Auto-translated
I started this campaign in 1.5. Before that, I completed it twice in 1.4 and 1.41. I wanted to find out when the red faction loses its patrol immunity.
The first time (1.4), I defeated him in the Castle because he found it before me. The second time (1.41), I attacked him as soon as he went down into the Dungeon.
In 1.5, I wanted to do this on the surface, across the river. But it didn't work. He didn't run underground when he crossed the river but continued to collect resources. He stayed standing right on the bank. When he collected everything, I approached him closely. He didn't pay any attention to me. I saved the game and attacked him. I watched a cutscene with a disappointed Markel, and then I received a message that the mission had failed. I don't think I had this cutscene in 1.41. I reloaded and went into the Dungeon. I was already approaching the Castle, and he was still standing on the bank. I don't remember if he stood on the bank for a long time in 1.41? I remember that he didn't decide to go underground for a long time.
Since he didn't attack me, it's likely that he can be defeated as soon as he moves. I won't check anymore. At least on the other side of the river in version 1.5, you don't have to be afraid of him.
Added after 22 hours 56 minutes
When I was near the Castle, the red faction went down into the Dungeon. I took the Castle, built a level 1 Mage Guild, and confidently went to defeat the red faction. But that wasn't the case. The cutscene with the disappointed Markel appeared again – the mission failed. I had to replay. After taking the Castle, I immediately rushed to the exit. I barely made it through the turn. The red faction didn't chase Markel but turned towards the Castle. He still wouldn't have caught up.
Conclusion: in version 1.5, the red faction is dangerous everywhere.