The loading phase can be one cause. For others, it might also be a problem related to Live Coding:
Structs (and sometimes also Classes), when compiled through Live Coding, can run into issues. It should work, but Live Coding is still a bit buggy from time to time and Patches don’t get properly reapplied when opening the Project again, most of the time a build through the IDE instead of Live Coding will fix it, but sometimes the Intermediate folder needs to be purged.
Funny enough, the Loading Phase is sometimes even an issue with Unreal Plugins. As far as I remember, the Audio Capture Plugin would sometimes throw Errors about missing structs through UE 5.1-5.3 with every restart. If I remember correctly, it was an issue with the loading phase. So yeah, it’s sometimes really annoying.
Hopefully, it helps someone at some point in the future 