Just wanna chime in and say: in the most recent update (7.0.0), this is still an issue. Frontend is set to 8080 on my Server, but it still claims port 80 on some boots.
Edit: i forgot to mention: the only reason i know this, is because Portainer actually prints a reason when a container fails to boot, instead of Unraids ominous "Internal Error", which tells exactly nothing.