Learn English – What are “crutch words”

It seems they are called Unpaired Words (maybe the best definition) or Absent Antonyms.

Unpaired Words
An unpaired word is one that, according to the usual rules of the language, would appear to have a related word but does not. Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite. Many unpaired words are the result of one of the words disappearing from popular usage, though others were never part of a pairing and just begin with the same letters as used in common prefixes. The classification of a word as “unpaired” can be problematic, as a word thought to be unattested might reappear in real-world usage

You can find some examples here, but I'll list the majority of them here anyway for easy reference.

1. Words with no positive forms:
   Debunk; defenestrate; dejected; disconsolate; disdain; disgruntled; dishevelled; dismayed; disrupt; feckless; gormless; impetuous; impromptu; inane; incessant; inchoate; incognito; incommunicado; indomitable; ineffable; inept; inert; infernal; inhibited; insidious; insipid; insouciant; intact; invert; misgivings; misnomer; nonchalant; noncommittal; nondescript; nonpareil; nonplussed; unbeknownst; ungainly; unswerving; untold; untoward.

2. Words with uncommon positive forms:
   Disarray; disconcerting; immaculate; impeccable; inadvertent; incapacitated; incorrigible; inevitable; innocent; inscrutable; insensate; insufferable; interminable; unbridled; unflappable; unfurl; unkempt; unmitigated; unrequited; unruly; unthinkable; unwieldy.

3. Suffixes (asterisk means "word not existing"):

   • Reckless/*Reckful
   • Indefatigable/*defatigable -> fatigable
   • Flammable-Inflammable (not antonyms)

As Greg said in his comment, it is important to remember that they are different words even if they happen to share an English spelling. This should help you to come to the definition of what to call them.

These are called homographs that are heterophones. Derived from homo (same) -graph (write) and hetero (different) -phone (sound).

A homograph (from the Greek: ὁμός, homós, "same" and γράφω, gráphō, "write") is a word that shares the same written form as another word but has a different meaning. When spoken, the meanings may be distinguished by different pronunciations, in which case the words are also heteronyms.