[Закрытые] URL slug for each language changing automatically

Background of the issue:
I have a calendar plugin FullEvents (скрытая ссылка). In WPML, I've set the slugs to be /evenement/ in French and /event/ for English. It works... until it doesn't. About once a week, these slugs change (I think they either switch or it changes just for one language) and create 404 errors. Temporarily correcting the issue is not complicated: I empty the cache and resave the permalinks and everything works again. But of course, I don't always catch this error as it happens, so visitors encounter a lot of 4040 errors. I would like a permanent fix for this.

Symptoms:
URL slugs for each language change automatically, causing 404 errors.

Questions:
How can I prevent the URL slugs from changing automatically?
Is there a permanent fix for the slugs issue in WPML?