It shouldn't be related to the master keys (though enabling encryption could be another thing to try!).
Master keys are used for end-to-end encryption. If they're disabled, they shouldn't be used to encrypt notes.
If you're using the TestFlight version of Joplin, you have the option to share a log. To do this, go to Configuration > Tools > Logs > three dots at the upper right corner > share.
Alternatively, you could try a different sync target (e.g. Dropbox or OneDrive). If sync works with a different sync target, this tells us that this is likely a bug in Joplin Cloud. If not, it's likely a bug in Joplin Desktop or Joplin Mobile. (Dropbox and OneDrive sync will be much slower than sync with Joplin Cloud).
Thank you for helping us fix this!