Steps to reproduce:

• Open a world
• Go into /gamemode creative
• Fly into the air
• Now, run /gamemode spectator
• Double tap space/space bar, depending on your platform
• And finally, go back into /gamemode creative

Notice how you fell to the ground?
When switching back to creative, the game is supposed to remember that you're still flying.

Potentially what's happening: the game thinks you're somehow turning on/off flight by double tapping space/space bar in spectator, even though it's impossible to turn off flight in spectator, and you're still flying.