All Mekanism machines are based on a 1:8 ratio except the Osmium Compressor which is based on a a 1:8000000000001 ratio which means that every single time you use the machine it ends at 99.99% and jammed up. I have have played with this mod for near a decade (and I love it, I think Aiden made a mod for people who just love to have fun) but this specific issue is not new. I don't even care, I only need a few ingots for high level tech, but it just seems stupid to break your own mechanics and not have machines work correctly. Minecraft has infinite ore, if you decide to make anything more arbitrarily shitty...

Bro, why?

I don't remember exactly how this worked in the old system (though I do remember the issue you are talking about). I just tested this in 1.15, and had to make a small fix to if the exact amount was supplied it not working properly, but in the next alpha of Mekanism this should be properly fixed.

Closing this as 9.9.10 is now up and on curseforge.

Is there a plan to backport this to MC 1.12.2 or not really? Thanks for your time and the great mod!

No 1.12 is no longer receiving development, also the main reason it was as easy as it was to fix in 1.15 was due to how we rewrote our entire recipe system in 1.15.