Changes to OnlyOffice files are not auto-saved

We have trouble with OO documents: changes are not auto-saved!
changes are saved when we hit the “save” button. but fast mode does not auto save, despite displaying in bottom bar “all changes saved”.

export files is always showing last changes, while “download” button and file on server do NOT saves changes.

It is as if cache was not written on file.

Some ideas?

Nextcloud 26.0.3
OnlyOffice connector 7.8.0
document community server
different browsers (several users in team)

thank you for support and awesome work.