https://i.imgur.com/uQX7rGH.png
This is my Frozen Ire. Without it equipped (no weapon equipped), my Frostbite damage shows to be 400 on my character sheet.
If I equip, say, a wand with 180% damage over time, it goes to about 490 or so (I don’t remember offhand, but that’s about right).
If I equip my Frozen Ire, it goes to 713.
Testing on a dummy, I can conclude that the wand with 180% damage over time does slightly more damage with Frostbite. So this does indeed seem to purely be a tooltip error.
I don’t really know exactly what’s causing it. I tried equipping other items with the stats that my Frozen Ire has, and nothing else seemed to mess the tooltip up.