This also happened to me in a game the other day. Healing trooper got stuck behind the guardian and my opponent never noticed or tried to shoot it. Gave me lots of free healing. If this was something we could intentionally try to "setup", like creep blocking in dota to make it take longer for creeps to get to lane, it could be a fun quirk of the game. But the troopers seem to do it erratically.
The most I could gather is that when the trooper turns the corner on the lane at too sharp of an angle such that it hits the bottom corner of the "half stair" doodad cutout in the lane, that it then seems to maybe make them want to repath to the top of the stairs or something?
Match ID: 13194927
Where and when: Around 1:40 IGT in the Haze vs. Dynamo lane. Green lane, blue team, behind the guardian.