I had the same issue with pyside2 in my original streamdeck-ui PR (https://github.com/NixOS/nixpkgs/pull/177240). The first two commits add egg-info to pyside2 and shiboken2 to fix it. I tried the same trick with pyside6, and it seemed to work, but no such luck with shiboken6.
What is egg-info, and why do we need to add it to these packages? Erm, not a clue. Python build tooling is a mystery to me.
egginfo contains package Metadata describing its version dependencies etc… They are used by the package system to find out what is installed in your system.
I did not check the pyside6 package but do you mean that the file is missing in the package output?
There is no other packages using pyside6?