So as I was looking for an underground shrine in spectator mode for some experimentation I noticed that I fell through the world right as I switched gamemodes. It reminded me of the way you fall down in survival if you get outside the range of a vicio ritual and I realized that I had one running. After a few tries I managed to replicate the bug: If you switch into spectator mode (with commands or F3+N in creative) while within range of a vicio ritual, you lose the ability to fly, will slowly sink in liquids and eventually fall out of the world. It works fine after respawn or if you switch while out of range and then go inside of it.
Minecraft version 1.12.2, AS version 1.9.4, both singleplayer and multiplayer