This should be fixed in Mendix 10.6.4 according to the release notes. Try upgrading to this version.
https://docs.mendix.com/releasenotes/studio-pro/10.6/#1064
I hope this helps. Good luck!
Does it happen in a sandbox environment (free app)?
I have a support ticket open where words with for example 'é' are replaced by a question mark ('Privé' to 'Priv?'). This happens for string attributes, but also for enumerations captions. Maybe your problem has something to do with that as well.
I created a support ticket and they told me this:
Our internal team has informed me that it appears there is a bug in our Sandbox environments (Free apps) and they will investigate the issue.
Make sure to create a support ticket.