Learn English – Pronunciation of “Porsche” over time

For often Merriam-Webster marks the pronunciation with T with an obelus (÷), meaning

indicates that many regard as unacceptable the pronunciation variant immediately following: nuclear \nü-kl-r, nyü-, ÷-ky-lr\

The Random House dictionary says:

Often was pronounced with a t -sound until the 17th century, when a pronunciation without the [t] came to predominate in the speech of the educated, in both North America and Great Britain, and the earlier pronunciation fell into disfavor. Common use of a spelling pronunciation has since restored the [t] for many speakers, and today exist side by side. Although it is still sometimes criticized, often with a [t] is now so widely heard from educated speakers that it has become fully standard once again.

The Oxford Dictionary says:

Usage When pronouncing often, some speakers sound the t, saying /ˈôftən/; for others, it is silent, as in soften, fasten, listen. Either pronunciation is acceptable, although /ˈôfən / is more common.

Generally in English the addition of an inflectional ending does not affect stress. That is, the ending itself is not stressed, and stress remains on the same syllable as it is in the uninflected form. For instance, even though in morphologically simple forms, we don't get 3 unstressed syllables at the end of a word in English, the addition of the -ing inflection of the progressive aspect does not affect the stress, even when it is added to a verb ending with 2 unstressed syllables: interest/interesting, amnesty/amnestying.

Likewise, the regular English plural ending is inflectional, and its addition never causes stress to shift to the right, nearer the end of the word: parentage/parentages, Cavendish/Cavendishes.

Therefore, if the -ta is a plural ending and is counted as an inflectional ending, like the regular plural ending, one would predict that stress would be unaffected by adding it, and you would get stigmata ending in 2 unstressed syllables. That is a big "if", though, since I don't think stigmata is actually the plural of stigma in current English, and I never heard it pronounced on anything but the penult. So I think the stress you are asking about is archaic in English.