Hi!
I stumbled upon this annoying bug today on PS4.
I accepted the mission 'Upside Down', then during the character select lobby, we actually had a message telling us an antag had joined the game.
Thing is, the antag somehow failed to connect and we ended up doing the mission without one (automatic win after some time).
Got to the mission reward screen afterward and obtained gold and a weapon blueprint for Lycus (Spinning Coin if I recall). When I got back to the game's lobby, the game stayed stuck in during the 'awaiting response from Mercury Steam servers' state. I waited for like 10 minutes or so, then gave up and closed/restarted the game.
Thing is, by doing so the rewards I had obtained by clearing the 'Upside Down' mission were not saved. So I lost my precious gold, exp and my weapon blueprint.
I'm a bit sad and frustrated at the same time right now