This is the modlist I am using, using PolyLib with FTB Backups 2. Didn't happen in 1.20.4.
It seems like this was fixed before but it's now back on polylib-2100.0.3-build.159-fabric.jar

This should be fixed in build 160, you may have missed it because for some reason it does not have 1.21.1 support listed on curseforge.
You will need to modify a config setting, that version has a serverOnlySupport setting that disables the features that break server only compatibility.

@brandon3055 I modified serverOnlySupport to true, but an error occurs. I am using polylib-2100.0.3-build.160-fabric.jar.

Do we have a fix for this bug? It is really annoying

We don't really have a good fix for this currently as the PolyLib power handling for cross-platform (Forge and Fabric) requires reborn_energy for fabric and it's them that is registering the data component so its not an easy fix