Forge 12.18.3.2185
RebornCore 2.8.5.79
TechReborn 1.4.16.133

I wanted also to add a recipe to the Grinder with Minetweaker. Looking into the github I could only find code for the Industrial Grinder which is fine, but it didn't do anything.
Then I saw the commented line in its code, so it seems broken at the moment.

is it just not showing up in JEI but works in the machine?

It is not working. Maybe I made an error in the script. I tried for example that:
mods.techreborn.grinder.addRecipe(<techreborn:dust:32>, <techreborn:smallDust:32>, null, null, <chisel:marbleextra:7>, null, 100, 48);

I think it's because of that commented line:
https://github.com/TechReborn/TechReborn/blob/1.10.2/src/main/java/techreborn/compat/minetweaker/MTIndustrialGrinder.java#L32

Yep it is that. Easy fix.