Issue was solved. I’ll leave the deets here:
Short version
Don’t run MV from an exFAT or NTFS drive on Linux. Put it on your internal drive and it just works.
The actual cause
MV’s Steam library was on an external Samsung T7 Shield, and that drive was formatted exFAT (they ship that way from Samsung). Steam on Linux can’t write properly to exFAT or NTFS libraries, and MV can’t run right from one either. The editor would open but close the moment a project was loaded. Installing MV to the internal drive fixed it on the first try. Plugins, permissions, and the project location inside the home folder all turned out to be unrelated.
If you have an external drive in the mix, check what it’s formatted as before anything else:
findmnt -T "/media/YOURNAME/DRIVENAME"
If FSTYPE says exfat or ntfs, that’s your problem. Either install MV to your internal drive, or reformat the external to ext4 (that wipes it, so copy everything off first).