That's been our experience with Unity's asset bundle and backwards compatibility too. They put little to no priority on not breaking things, and so they just make changes to things like shader handling and expect you to rebuild bundles between versions. Saying apps Flipside Studio aren't exactly using Unity for what it was intended would be an understatement... :P

We did a bunch of work to try to auto-correct older bundles as much as we could when we moved from Unity 2017 to 2019, but weren't able to automatically fix every case.

When we update again, I'm assuming we'll have to fight with more of those issues then too, although at some point we'll have to contend with updating to the new render pipeline too (fortunately for us they're taking forever to get it up to parity with the old one) which doesn't support any of the old shaders. At that point, we'll have to maintain the old version of Flipside Studio for a while since all assets made for previous versions will break in the new one.

I don't mind that bundles are platform-specific, but it would be nice if they weren't also limited by Unity version too.