Confused about nation-switching incident

[SciencePro]

I am playing in a multiplayer game as abysia (operationanchorage). During turn 64 I ordered one of my arch devils to move from one friendly province to another. There were already some of my infantry in the province and I had an air queen move into that province during the same turn.

When I received turn 65, there was a battle message from that province. It says my forces were defeated by a stronger force from mictlan. When I watch the battle what happens is that my own arch devil (with the same equipment and custom name), by himself, attacks my infantry and air queen and kills them all. Now the province and the arch devil are owned by mictlan.

Is there some spell or ability that would cause the devil to defect to another nation while traveling from one province to another? Or could this be a bug?