I am seeing this for trinkets (
[Figurine - Twilight Serpent]) and 4 rings (
[Ring of Northern Tears] lowest) as well. It is not specific to weapons. These negative values appear to only occur with certain gear combinations.
Edit: Just noticed another
very strange case. It is showing
[Figurine - Monarch Crab] is worth 43.0 with 2x16 agi gems, but worth -108.59 with 2x27 agi gems. I'm also seeing values jumpimg around when I toggle "Enforce Gem Requirements" on and off, but between different values (not just back and forth between 2 values). It's also showing different results when I sort Alphabetical instead of Overall.
In all cases it looks like it is items with JC gems that are showing negative (with 3 already being used). So, I think I found the problem. When you equip more than 3 JC gems, all the gems become invalid. This means by equipping that item it is actually making other items worse. If the base stats of the item are worth less than the value of the other 3 gems, the item will be negative.