Due to an error in NeutroniumCompressorTickProcedure.java, inserting a netherite block into a neutronium compressor will display the material as Amethyst and refuse to accept any other netherite blocks. This is caused by the string here saying "material","Amethyst" instead of "material","Netherite"

Indeed. Confirmed this with my own copy of the mod running on 1.20.1 with Forge 47.2.19. Searched the code on GitHub and it appears on Line 1104 of NeutroniumCompressorTickProcedure.java. Looks like a copy/paste oversight. Should be a quick fix.
There is no workaround for this other than compiling the mod from scratch with the altered line in place, but you can cheat in the singularity and toss a thousand blocks of netherite to feel fair...

Sorry for late answer. This issue has already been fixed.