Technically working as intended. Further down there’s more on the subject. If you look at the penetration stat of the Latrine Shovel, the light attacks and heavy attacks have different flak and carapace armored penetration values, with the Heavy Attack penetration value having the potential to go more than 100% for both.
This means when you get Rending or you apply Brittleness - both use the same damage calc - the damage of your hit will get normalized towards 100% base damage depending on the number of stacks of Rending/Brittleness you have/the enemy has. This is why the Latrine Shovel’s light attacks gain damage (in this image, would get buffed from 88.28% to 100% at 20 stacks), but its heavy attacks instead lose damage (dropping from 116.24% to 100% at 20 stacks).
Yeah, not necessarily optimal, but that’s exactly how it’s supposed to work. I believe the devs have said they’re looking at this, but I haven’t heard an update on it recently. I hope we can get an update once they get back in office.