I'm not that keen on kernel device hierarchy, can it be that events kernel entry points rely on available inputs (which in case of the missing udev rule aren't available)?
It's the other way around - kernel input events are the lowest level, upon which higher level APIs are built (i.e. joystick or gamepad APIs).
What I am sure abt and can replicate it, is that without the rule evtest doenst show the gamepad as normal user, while it shows it as super user.
I cannot replicate it, but I'm not running the same configuration as yours (i.e. desktop environment and extra udev rules from MoltenGamepad).
One more think I've noticed is my raspios keep asking me to update sdl and sdl-dev library which were not automatically updated by apt ugrade, and that for some reason the retropie installation start from source and compiles everything, which it didn't seem to happen the first time I've installed it (but I'm not sure abt that)
That's ok and it's not a cause for your issue.