Bear uses iCloud storage which has on multiple occasions trashed people's data during iOS beta periods. It's not "Bear Sync", it's just another iCloud beta sync problem. If you value your data, don't use iOS betas. It's that simple.
If they had the choice, many developers would disable their apps on beta devices because of issues like yours.
It was during an iOS beta on one of the devices, so perhaps not their fault—but I’ve been unwilling to pay since.