OnDeserialization() is sometimes not called (on some or all behaviours) for late joiners. In limited testing I think that the likelyhood that it is not called increases with the amount of data stored in UdonSynced variables.

In my testing I found that this is easy to reproduce using manual sync behaviours with 2 "bunches" of data. I get client logs about the bunches being collected, but no OnDeserialization().

Here is a repro world where this behavior can be witnessed. It needs 2 players to test:

Because the bug is intermittent, the 2nd player may need to rejoin many times to trigger it. In my testing, it is usually triggered between 2-20 joins.