I wonder if this is related to some issues we had way back in 10.2.x. We had some business logic in a UI customization (bad practice) and when we refactored it, we removed the UD columns it was using. From that time forward, users would randomly revert to the old version of the UI that referenced the now non-existent UD columns and get errors. We’d delete their client cache and it would fix the problem for a while. Weeks or months later, it would magically revert again.