BTW. I have made a few experiments and I guess these observations may help to finally solve the bug.
In short: this bug is 99% dependent on "SpellTargetType" and the way how location of the summoned unit is calculated for AI.
What is important, that different combinations of: 1) SpellTargetType 2) SpellTargetTileOccupied and 3) Radius affects the result and produce the variety of outcomes.
Major outcomes for AI are:
Outcome 1. Spell is cast, but the unit is not summoned at all, and then battle flow continues normally. This is what is happening with "Summon Darkling" for example.
Outcome 2. Spell is cast, but the unit is summoned SOMEWHERE IN ANOTHER DIMENSION (joke). You can see unit card in the actions list but not the unit on the battlefield. This outcome will break the normal flow of battle, summoner will be waiting forever for something. The only way to get out the battle now is autoresolve.
Outcome 3. Spell is cast, and produce SOME results (described below). This will may or may not break the flow of battle, which makes me think that there are at least TWO parts in this bug, and something else is responsible for the freeze of tactical battle.
Now, what you can do: if you apply radius (e.g. the same way as in "SummonIgnys_Tactical" or whatever spell with area effect) and remove ApplyToSpellRadiusCenterOnly tag, the spell WILL work for AI, however now of course it will summon up to 8 units instead of 1. I guess that ONE unit still may be randomly "summoned in some bad location" as in Outcome 2, and therefore freeze, however it's only a hypothesis.
And finally very unexpected Outcome 4.
Spell is cast SUCCESSFULLY, and battle flow will likely to continue (at least I did not observe the freeze in my test).
However, the location of the summon will be ... wierd, and this outcome is a result of a parameters WHICH ARE NOT SUPPOSED TO WORK. So while it may solve the problem, I would not recommend anyone to modify the XML in such a way.
The trick is that if you will change SpellTargetType value from "EmptyTile" or "FriendlyUnit" or whatever valid for tactical combat to "AnyKnownTerritory" which SHOULD only be valid for strategic map, then AI will call the summon on top on one of your units, but everything else will happen OK.
That means that TECHNICALLY you can solve this by the changes in XML, but as i said this NOT something you should do. I only hope this information will help SD to find where the AI fails in calculations.
P.S. As for the freeze you can reproduce it for the player as well Just set SpellTargetType to "Self" and even with enabled radius you will get Outcome 2.
All in all it seems that AI somehow wants to makes a summon as close to your units as possible (ideally - directly on top ) but when it fails to do so, summon will either not happen at all or happen in some really, really bad location.