Discussion about Dungeon (Dark Elves).

I wasn't talking about witches, but in general. I was playing as the Tower faction and divided the Assassins into several groups and placed them next to each other. I didn't have any Minotaurs in my army. I just noticed that before the battle, the morale was +1, and during the battle, it became +2, although it's possible that I simply visited a +1 morale shrine and didn't notice. So, I apologize for misleading everyone.

You didn't mislead anyone. And you wrote correctly above, with the clarification that it's not just when they are next to each other, but in general:

friendly units have +1 to morale when they are standing next to each other during battle.

Morale depends on the army composition in battle. So, if troops from the same castle are fighting in the army, their morale increases by +1. If the unit stack is one, then the morale remains unchanged, but if there are already 2, then the morale will also increase. That is, to increase the morale of the Griffins, if they are the only ones in the army, for example, you can simply divide them into two stacks (30+1, for example).
The same applies to witches and other troops. If there are only witches in the army, their morale will be +1, but if they are divided into two squads, the morale will be +2.

Very often, Minotaurs in the Warlock's army may not be present and are only bought for the final battle.

Or they are not built or bought at all ;)
I rarely even capture their dwellings.