Thanks for making such a fun game. I just finished a playthough on the largest map with every faction. So i had a few more minor bugs to point out.
There was one time where I had my knight diagonally positioned on the left to access my castle for recruitment, on the AI's turn they moved to the point diagonally to the right of the castle port, when I accessed the castle, my knight was no longer in the bottom slot, so I couldn't reinforce the knight further, or get the knight to be defended in the castle walls.
I suspect you're already familiar, but in rare situations sometimes there are issues with the large battles ending. I had a necromancer in a castle who was able to rez skeletons consistently enough that he managed to destroy all of the enemy units, but all of his original units had died, this resulted in the battle being declared a victory, however the knight was dead, and the castle was lost, and the AI lost their knight as well. In a similar situation I red was battling blue, my original units were dead, some spawns were still alive, and somehow i had some units that were not my race (i dont know how i acquired them during battle), the blue bar for the AI, spread horizontally across the screen farther than its supposed to, but defeat was not called for 5-10 min, my surviving spawned units did not die, and the enemy units seemed to dance, and occasionaly there were attacks and explosions but i don't think they were directed at my summons.
Minor bug, I had the terrain randomizer on, the neutral border unit was passable diagonally early allowing for an early second castle, I've noticed the AI take advantage of this several times, I think the easiest solution would be to increase the unit area of attack on the border units.
On my play on the biggest map, actually I expected to lose, the AI had a much more powerful army than me about 3 times as many units and more castles. However it made many mistakes. I noticed after the borders are down the AI stops ever attacking neutral units, it makes sense they are mostly a waste of units at that point. But it also meant they stopped attacking the neutral border units and did not perceive threats there, so they left their rear castles defenseless, further as I attacked those castles one by one, they did not defensively attack me, they focused on attacking me from only one front. Then as I had more and more castles I had more knights, I always have 1-2 per castle, The AI did not do this, mostly having less knights than castles. It resulted in them not capturing resource points. I regularly controlled resource points in enemy lands for weeks without the AI bothering to capture them.
Regarding the unit stacking, I wonder if it would be reasonable for you to make it and other performance tweaks like max units on the map options that the players can pick in order to eliminate lag or have insane battles as they like.
Also with the premium edition I had several crashes in a play through, all draw errors, i lost the screenshot.
Btw, my favorite unit is the beholder, do you have the stats for those win rates public somewhere?