ref. CleanroomMC/Cleanroom#159

Cleanroom is a Forge fork that allows running 1.12.2 using Java 21, however, since this mod is targeting Java 8, some features just don't work (like "Suspected Mods"), along with random errors during launch (https://mclo.gs/GXxH4Dr)

Cleanroom usually resolves these incompatibilities via their Fugue patching mod, but in this case they refuse to do so for FermiumASM because:

Just use loliasm, [...] Fugue won't patch the older/worse version of a mod"

(direct quote from the above linked issue, for context, they have patches for CensoredASM fixing similar issues)

Would it be possible to add direct compatibility with Cleanroom in FermiumASM itself, since the Cleanroom team is not willing to patch the mod?

FermiumASM is not outdated, nor is it identical to CensoredASM regardless of name, nor does using FermiumBooter mean you can't use MixinBooter, but if they want to choose to be misinformed then that is their choice.

The latest FermiumBooter update fixed launching compatibility with Cleanroom and from my testing you can use it alongside FermiumASM, just with disabling some of the config options.

As for other compatibility issues like the one you linked, I'll take a look at fixing them, however it may be a bit as I have other priorities currently.