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Yuri commented
I have the same problem on Windows 11 and Desktop Illustrator. For me, the diacritical marks over Cyrillic letters vanish: if I paste a text string containing и́ (that is и + ◌́), I obtain и, even if the font can display the combination perfectly well elsewhere. It also occurs with other diacritical marks even with Latin-alphabet letters. As well, there is a duplicate issue (”Diacritical characters are displayed incompletely”) stating the same essential problem that even includes a workaround in the comments.
A simple solution involving selecting the “Show Indic Options” (as detailed on https://www.behance.net/gallery/124447049/Javascript-for-Illustrator-ShowAnyGlyphs) solves the problem.