Thanks for the detailed report! I believe that was a bug introduced while implementing the Mageflare Rune and Gloom interaction. I'll have a look at that.
The normal Mageflare Rune behavior in zones outside of Umbra is actually a separate mechanic that fully removes darkness, rather than just boosting light radius by a large amount. For the Gloom debuff specifically, I created a different effect where it adds +300 light radius on top of your character's existing light, but that extra light is handled as its own instance, so it does not directly modify the base light radius value (therefore not interacting with all light-based mechanics in the game), and that is intended.