What you were expecting to happen, and what actually happened:
During the delve battle, i activated Zuul Goths ability and was unable to pick any troop as target, all opponents troops were shown as stealthed/not a legal target
How often does this happen? When did it begin happening?
I got it once 14/10/22
Steps to make it happen again
Can not provide any information on this, I did not notice anything unusual in my game activity, got the bug only once
EDIT looks like that has alredy been reported several times
Burning Gems in play? Yup, it’s definitely been reported many, many times.
What happens is: any time you initiate a manual-target spell, if there are any of these special gems (Burning, Freeze, Curse, Lycanthropy, etc.) actively floating across the board, when they hit their target and inflict the status effect, this seems to remove that Troop from the list of available targets, with no way to “refresh” it.
In your case, notice that Crimson Bat – the only non-Stealthy Troop on the enemy team – is Burning. I’m willing to guess that a Burn Gem touched it approximately the same time as you had Zuul’Goth start casting, which (as described above) created the bug.
The developers fixed a similar issue involving Submerged Troops and Ironhawk’s third trait (which damages all enemies on spellcast, “all enemies” thus triggering Submerged), but this bug remains – maybe it’s more deeply rooted and harder to truly solve.