When you run /gamemode 1 @a using a player or a command block, it gamemodes you, allows you to fly, but doesnt prevent fall damage.
Linked issues
duplicates 1
Comments 3
I don't know, but it happens. I also just found out it only happens in multiplayer, might only for a LAN world.
Duplicate of MC-44521 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.
How do you get fall damage in creative ?