I cannot get a useful stacktrace.
This and this are reports to a similar problem.
I assume that no system changes have been made and the package kwallet-pam is installed.
If it is a general openmamba problem it should be reproduced by me or reported by other users when updating existing installations or doing a fresh installation but the fact that this is happening only to you and on two installations suggests to investigate on any post installation operation you have made on both systems, for example the installation of any external software might have changed the /etc/pam.d files and as written above it seems that the problem is at PAM level.

I believe kwallet-pam is installed by default.
The system I am using right now has some non-default software installed, like nvidia_470, though that shouldn't touch kwallet.
However, my girlfriend's secondary system is much simpler: it's an intel-based laptop with no discrete graphics and no additional software installed.
The issue appeared immediately after install too, before any updates, and persisted after updates.
Should I provide a stack trace using the same method on her laptop? I can do that later today. Let me know if there is more I can provide so I only take over her workstation once today

I will do an installation test with the latest livecd ISO image. Did you set automatic login on your installations?

No automatic login. Disabled root user login.

I've tried today's livecd, installed the system on VirtualBox, rebooted, logged in, run firefox and did not get any kde wallet password request. I also tried logging in with plasma-x11 and did not reproduce the problem either.