Hey folks. I stumbled on this thread and wanted to add my observations as we are experiencing the same behavior the OP describes.
We are doing some testing with Realm Sync and Unity as it seems like an attractive option for a future project.
Currently using 10.1.4 of the Realm Unity SDK and seeing the following:
–Logging in with emaill/pwd creds, syncing, and making changes seems to work as intended the first time after opening Unity and pressing play
----A second play always freezes the editor
–Based on what I see in the documentation (and some experimenting) it seems we need to specify a filename when we call GetInstanceAsync to get things to work as intended rather than a new .realm file being created on each sync and a resulting resync of all data
----This works properly if I only specify a filename (subsequent syncs no longer show ‘for the first time’ and current data is not resynced
----If I try to append a path (to the persistentDataPath folder for example) the editor crashes when trying to sync whether the file exists or not
–Testing falling back to offline mode (via Realm.GetInstance) after a successful sync doesn’t seem to work
----I receive ‘Incompatible histories. Expected a Realm with no or in-realm history, but found history type 3’ when attempting to open the .realm file
Hoping this is useful in some way and wondering if the bug-fixes mentioned above were included in 10.1.4?
Looking forward to efforts on this continuing as I think it would potentially be a great option for Unity developers and would like to give it a more through test-run myself.