I have a custom entity in an Organization that was created from scratch in CRM 2016 (on-prem), so not migrated/updated/anything. In one custom entity, we have a business process flow that seems to break the Information form unless I turn on the Legacy rendering. It just sits there at the "Updating" circle. (This is different from most issues I've seen on here, where it gets stuck at Loading or Loading Business Logic.) This BPF was made in 2016 itself, so I don't know why it would break anything? I'm getting the same behaviour even if I remove all Javascript from the form, but it goes away when I Deactivate the BPF. That is, when the BPF is deactivated, new ones work, but old ones that were created with that BPF still won't load.
The only thing I can see so far that's a clue, is this post: https://community.dynamics.com/crm/f/117/p/178159/488916#488916 Someone else was having an issue with forms getting stuck, and this comment references an issue with a BPF that's multi-entity, which ours is. I can't quite grasp what exactly the issue is with a multi-entity BPF or how to fix it, though, from the few details there.