Creatures can have higher level than 10
In "Behind Enemy Lines" campaign level, I think it's level number 6, spawning dwarves gradually increase levels if you take your time. Eventually I got lvl 11-13 dwarves before completing the mission. I saved the game once since it took about 2.5 hours to seize the whole map, so loading it might cause it, I suppose.
I tried hard to replicate this one, but for me it works totally fine... :<
Could you please upload your output_log.txt
It's under: D:\Steam\SteamApps\common\War For The Overworld\WFTOGame_Data