It is multiplayer bug
Once I was the host, the other time @ember tree was the host
Bug: When joining a lobby when they are already in game and then the host dying it can lead to a bug where you can fly. (pressing WASD will lead to walking forwards, backwards, right or left (depending on the button), holding left-click and moving the mouse will lead to looking around, pressing control will lead to flying down, pressing jump will lead to flying up). It will also not fully restart the level and leave the level in a buggy state of being frozen in time.
How it can be recreated;
The host needs to create a game and start, afterwards the 2nd player has to join, then the host needs to die in the pitfalls of level 0 (i don't know if other ways of death count)
Videos (I'm sending them using google drive cause they are too big to upload):
https://drive.google.com/file/d/1Ensz1_-Q4b7zdzxEdyLqGPJDn0J7NU3f/view?usp=drive_link
https://drive.google.com/file/d/1QjQPoNww7pMWdOmrCZtlSFJFiaUBRovC/view?usp=drive_link
(also I might be AFK for some seconds, that because I was chatting with @ember tree (my teammate) using discord)