I am afraid that I am late with an answer but here it is.
You use test app id = 480. Many other people use it for testing Steam integration in their games as well. When you ask Online Subsystem to look for sessions it actually finds those other games sessions. Your game fails to read those incompatible sessions data so it does not return them from Find Sessions node. Engine logs on the other hand are full of attempts to parse those sessions.
I would expect that Find Sessions->Max Results parameter limits the number of compatible sessions to find. But actually it limits total results number indifferently to compatibility. So when there are 100 other game sessions online and you call Find Sessions with Max Results = 10 good chances are that you won’t find the session of your game at all.
Quick fix is to increase Max Results to some greater value, say 500.