When using Cyrillic in the code of the quests I used UTF-8 encoding, but after starting the game mod changes it to Windows-1251. At the next launch an error occurs and the quest file is cleared.

But at first start up everything works fine, displays the Cyrillic alphabet normally without errors.

Is it possible to fix this or somehow avoid it?

Sorry for the late reply, only seeing this now... AFAIK, all UTF8 should work(besides some dodgy behaviour around escapable chars (like backslash, newline, carriagereturn, tab, singlequote, doublequote, etc)). Ill try reproduce your bug soon, and keep you updated on it here (and/or in the discord if you're in there). The only workaround that i can think of right now that will most likely work is that you use 'customquests:translation_key' instead of 'customquests:plaintext' as texttype and add the translation key of your choice in the text field, and then add a Cyrillic lang file to a resource pack to the game. (this also allows for questing with localization btw ;) )