Hello!
As @Dust_Angel and @Tabu have pointed out, this is not a bug.
The reason this interaction is in Gems of War and the Nature’s Aura talent hasn’t been changed on Sunspear to avoid this clash with its Firestorm Trait, is due to how the Talents in the Talent Trees are available across multiple Hero Classes.
As the Talent Trees for Fire and Forest are used by other Hero Classes, we’re not easily able to change these Talents without effecting other Hero Classes and how they are designed to work.
We did at one point change the Firestorm Trait on the Sunspear Class to prevent this clash from happening, however, due to feedback from the community this change was reverted. The forum post linked by @Cairnso explains this in detail.
OminousGMan - Support Human 