Hello,
you have the “Sync global classes” setting enabled, right? There is a known error, causing changes in class to be reverted/gone, if you the other user saves a page, that has the same class applied.
We have an internal bug report for this. Please note, that “Sync global classes” function is experimental, and intended mostly for testing and/or working on a staging website, not on live.
Best regards,
Matej