The Wilding Hermit has 4 Abilities. As you've pointed out, Coalstones and Summon Wilding are working as intended. There are issues with Counterspell, but it is able to use it. Summon Ignys (Ability) on the other hand, isn't working for the AI because of how it's set up. It's fixable through Mods. At any rate, Summon Ignys shouldn't prevent the Wilding Hermit from moving as the AI doesn't use the Spell at all.
What likely happened is the Wilding Hermit had already used Coalstones/SummonWilding (Both of which have a 5 Cooldown), couldn't use Counterspell (Since you were not casting), and couldn't use SummonIgnys (Because it just can't use it). In other words, it had to move in for an attack and it couldn't move in because there was no empty tile reachable. Now here's the tricky part...
The AI selects a target before it moves. It then tries to move close to it for the attack. If it's interecepted on the way by another enemy unit, it will attack that unit instead. If it's not interecepted on the way, it will move in for the attack. However, if its target is unreachable (say corner/bottleneck and/or no empty tile available and/or no way to reach the empty tile because doing so would mean stacking units on the same tile), it then goes for the second best option tile. However (bis), if the second best option tile is also occupied (by another unit which went through the same thought process), then the unit doesn't know what to do and combat stops. It doesn't seem to go for third/fourth/etc. best option tile.
The only two ways to currently fix this is to (1) use auto-resolve as you did or (2) use CTRL-T to teleport the stuck unit on some other tile (Requires Cheats enabled). Using (2) allows the battle to continue as the stuck unit will be able to use its movement points. You have to put your mouse cursor on the destination tile and then press CTRL-T for this to work.
tl;dr
Probably an issue w/ the Tactical AI rather than an Ability. Use CTRL-T (Requires Cheats enabled) to fix.
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XtraDeconstruct
an LH Mod by Primal
