Just as 〈ng〉 makes /ŋ/, so too 〈gn〉 can make /ɲ/

As you’ve discovered, poignant and its derived forms poignance and poignancy are pronounced with a different sound than occurs in words with similar endings like benignant, indignant, malignant, oppugnant, pregnant, regnant, repugnant, resignant, and stagnant. Those others all have /gn/, but poignant alone does not: it has /ɲ/for same spelling.

For the word poignant, the OED2 gives two phonemic forms: /ˈpɔɪnənt/ and /ˈpɔɪɲənt/ It is the second of those that has drawn your attention. I have not heard the first of those myself. Note that Oxford Dictionaries Online now gives /ˈpɔɪnjənt/ instead, swapping ɲ for nj. Those are really just different ways of writing the same thing, at least at the phonemic level in English (and finding minimal pairs in other languages is far from easy).

English usually spells the /ɲ/ sound with orthographic 〈ni〉 or somtimes 〈ny〉, but unassimilated loan-words often retain alternate orthographic representations for that sound, and poignant is one such case.

What you are hearing there is the palatal nasal sound, which is represented in IPA as ɲ. It is made by pushing the flat, front part of the tongue up against the front of the hard palate.

This sound occurs in many languages besides English, but its orthographic representation varies widely. A few of the more familiar might include:

• In French and Italian words, it is represented by the spelling 〈gn〉.
• In Spanish, Galician, and Basque words, it is represented by the spelling 〈ñ〉.
• In Portuguese and Occitan words, it is represented by the spelling 〈nh〉.
• In Catalan and Hungarian words, it is represented by the spelling 〈ny〉.

So what’s happening with poignant /ˈpɔɪɲənt/ in English is that it still (usually) follows the French rule for the 〈gn〉 spelling — although not for the rest of the word: in actual French, it would be pronounced /pwaɲɑ̃/ instead.

Details

If you look at common English words like onion and canyon, you will find the sound there, too, although many dictionaries represent that as phonemic /nj/. When phonemically spelled that way, this sound is quite common in English, as those words show. I’ll let someone else comment on the differences between [nj], [ɲ], and [nʲ] if they care to; I’m not convinced those represent distinct phonemes in English, and rather suspect all occur allophonically.

You will notice that both onion and canyon were originally loan-words themselves, respectively French oignon /ɔɲɔ̃/ and Spanish cañón /kaˈɲon/. The English spellings have discarded or adapted the original orthographies in favor of things English-speakers are more likely to recognize and reproduce.

But note the American city of Cañon City, Colorado, which has retained the original spelling. That’s because proper names like cities tend to be conservative in their retention of older spellings, rather like how Pittsburgh still has a trailing h where most other towns founded since then no longer do.

Other /ɲ/ words

Nonetheless, the OED attests many words with the /ɲ/ phoneme occurring in them, virtually all of which are themselves loan-words of one or another sort. The orthography generally remains as it was in the source language, or a Romanized version thereof. Here are just a sampling of a few of them:

araignée, assignat, bagne, beignet, Bolognese, box-cañon, cache-peigne, cagnotte, campagna, campagnol, caña, cañada, cañon, caraña, carmagnole, chignon, chuño, cognoscente, compagnon de voyage, consigne, coynye, dal segno, dame de compagnie, Dom Perignon, Doña, El Niño, encoignure, espagnole, español, española, farinha, garconniere, gnocchi, Guignol, ispravnik, jalapeño, La Niña, lasagne, limeña, Limenian, limeño, lorgnette, lorgnon, Luiseño, macigno, Madrileño, madroño, magnanerie, Malagueña, mañana, mignon, Mimbreño, Montagnais, montagnard, montagne russe, música norteña, Musigny, Narodnik, norteña, norteño, Orcagnesque, ordoñezite, pagne, Peau d’Espagne, peignoir, pignon, piña, piña-cloth, piña colada, piñata, piñon, piñon jay, piranha, poignance, poignancy, poignant, Porteño, pregnada, provodnik, quinceañera, recitativo, renierite, renseignement, rognon, rondeña, Sauvignon, Seignette, seigneur, seigneury, senhor, senhora, senhorita, señor, señora, señoría, señorita, sevigne, signifiant, signifié, signora, signore, signoria, signorina, soigne, tagnicati, tortilla española, Valdepeñas, vargueño, vicuña, vicuña-cloth, vicuña-wool, vigneron, vigogne, vinho, zampogna, Zuñi, Zuñian.

Most of those are from Romance, and are spelled per the rules given above. A few are from Russian, though, like ispravnik /isˈpravɲik/, and Russian follows very different rules regarding palatalization of consonants in certain positions.

So what you are hearing is simply someone using the pronunciation rules that correspond to that of the source language. Surely lasagne is not a rare word in English — and note that the Spanish spell that same word lasaña to match their own orthographic conventions. That’s like how for the word that the French spell champignon (meaning mushroom), the Spanish spell champiñón: different spelling rules for different languages but the same underlying sound, and indeed the same word entirely.

The /ŋ/ sound

A closely related sound is the velar nasal sound, ŋ. This sound differs from the previous sound ɲ in that now the back of the tongue pushes up against the soft palate instead of the front of the tongue against the hard palate. It regularly occurs in English words like singer, finger — as well as banker, ankle, anchor, function, uncle, encourage, conquer, and inquisitor, and in such loan-words as cinco and bronco.

Note however that it is not guaranteed whenever /n/ and /k/ collie; for example, it does not occur in concur, concave, cancan, mankind, downcast, bean-cake, moonquake. The conquer–concur pair is especially interesting; the rest are at morphemic boundaries.

Sometimes this effect is phonemic and sometimes it is not: most anglophones don’t even notice they are doing it whenever phonemic /n/ precedes phonemic /k/, because this sort of regressive assimilation is extremely common, and not just in English alone.

A related example of this is how the English word emphasis while phonemically /ˈɛmfəsɪs/ normally becomes [ˈɛ̃ɱfəsɨs] phonetically.