Does it happen if you try to save while selecting the objects on plan view? 3D views doesn´t allow to save objects as custom parts.

It’s was magnet all along, the bug is gone!!! Soo many years with this problem on both m1 ultra and M1 Max. I will just use the new window management on new macOS sequoia. Thank you, you made my day!!!! 

I have this bug and I have magnet installed. I will uninstall it and see how it goes. This bug is driving me crazy.

This has been driving me crazy for like two years. Nice to see more people with the same problem.