Scenario:
1. Join a server
2. Click "Esc" on the keyboard to see the menu
3. Choose "Unspectate"

Result:
Screen turns black
(Note - spectate then unspectate again didn't fix it, neither did "/k (player number)", only "!k" did)

Video attached:
https://streamable.com/8t6ewi

Thanks for the new discovery! This is potentially fixed in 6.0b but I'm leaving this thread open for now... this is a complicated area of code and this issue would only happen occasionally to begin with, so it's hard to be sure whether it's completely fixed.