I’ve read the comics but I don’t remember if they explained it there, but in the show while on the thraxan planet he tells Mark that viltrum knows he abandoned his post, how does he think they found out? He was already gone by the time Allen revisited earth so he wouldn’t have a way of knowing that Mark told him he was gone would he?
Maybe he was supposed to make reports back to viltrum periodically. Or they would request updates, or even just idk call in to check he’s alive. He was gone for like 6 months which isn’t much on the Viltrum lifespan but from a military perspective it would make sense if there’s a periodic report