It’s either being added into Frostbolt damage or not being registered at all. I cannot confirm as I reset my damage meters. I did not notice this before, but it seems like it’s been like this since at least Nighthold, so I assume it’s being added into Frostbolt damage. Just a small thing if you want to take a look at it. QoL to see how Unstable Magic performs standalone. Thanks.
Click the arrow next to Frostbolt to open it up and see Unstable Magic. Working as intended.