systemd version the issue has been seen with

245.r819 onward

Used distribution

Manjaro

Expected behaviour you didn't see

Normal boot when SecureBoot is enabled.

Unexpected behaviour you saw

When SecureBoot is ON, there's the following error:

Failed to fix up PID 1 environment: No such file or directory.
Freezing execution.

Steps to reproduce the problem
I observe this since #15640, which was meant to be a fix for #14864, but unfortunately it caused a disruptive side-effect described above.
Sadly I couldn't find anything in the journal.