Testers needed.

Did the mage appear before or after completing the prophet's quest? Mages shouldn't leave the garrison before the quest is completed.
The size of their armies is also strange; they couldn't have gathered 10 dragons and 9 thunderbirds in a week, and I didn't give them any.

The mage (orange) appears almost immediately after Andreas on the field in front of the castle. (The event depends on how quickly Andreas appears near Fortress. The mage might intercept Andreas on his approach, somewhere near the fort on the hill). Usually, this happens towards the end of the second week. You can't approach the hut; the mage blocks the road – the quest hasn't been taken yet. Different mages appear, and their armies differ slightly, but are generally the same. The purple mage behaves strangely: when moving towards the castle, he lets Andreas pass; the main thing is to find a place where they can diverge. Then he returns and chases him until the end, until he corners him. For clarity, I'm attaching the latest save of the map and a replay of the battle with the purple mage (I attacked him myself, to assess his army; it's been 1 week and 1 day).