Portal: New games periodically stuck on empty map screen, can't deploy or quit

I'm on PC (Origin). I've been trying to play several of the Portal playlists this evening, including the official "Conquest of Ages" and a few custom games. Every few rounds, when the new round starts, it loads the map/deploy screen, but there is nothing on it. Just the raw map. No spawn points; no objectives; not even the menu. It's just blank. I can't spawn in. There is no menu, so I also can't exit back to the lobby. I have to force quit the game window.

It happened to me 5 times tonight. It's doesn't seem to occur in the first round, but may occur in any of the subsequent rounds in the same lobby. Here are screenshots of two of the occurrences:

Conquest of Ages - Battle of the Bulge

Custom game - Noshur Canal

It just sits on this screen like this. The rest of the UI never loads. I can see movement and hear sounds in the underlying map, but I can't do anything. I can bring up the console menu (~) but that is all.

I haven't played Portal too often but this is the first time I've seen this kind of thing happen. It doesn't happen on the official AOW modes.

Has anyone else been experiencing this issue? Any fix?

Edit: Adding my PC Specs just in case. Nothing leading edge.

CPU: Intel(R) Core(TM) i7-8700K CPU @ 3.70 GHz 3.70 GHz
RAM: 16 GB (2 8GB DDR4 2400 by G.Skill)
GPU: Gigabyte GTX 1080