The bug

If the player is in lava when in creative mode, uses spectator mode to exit the lava, and switches back to creative, they are on fire for a very short amount of time.

Steps to reproduce

This is easy to do in the Nether.

1. Go into creative mode
2. Enter lava
3. Go into spectator mode (F3+N)
4. Exit the lava
5. Go into any other gamemode
   : The player is on fire for a short amount of time in Creative, while on Adventure and Survival, the player is properly on fire.

Possible cause

Normally, when a creative-mode player goes into lava, the Fire value of their NBT is set to 0, causing flames to overlay the screen. When said player steps out of the lava, their Fire value is reset to -20.

However, if a player is in a spectator mode, the Fire value is not updated at all. This means that if the player is 'on fire' when they switch to spectator mode, they stay 'on fire', even if they are out of the lava. (The flames don't show because, spectator mode.) This means that when the player switches back into creative mode, it takes the game a short amount of time to realise that the player isn't on fire, and reset the Fire value.

This same delay is present if you simply step out of the lava when in creative mode, but is much less noticeable.

I have attached video 1, where the Fire value of the player is output to chat every tick. It shows the bug is reproduced.

I have attached video 2, where the Fire value of the player is output to chat every tick. It shows the bug is reproduced backwards. Note how there seem to be three ticks after switching to creative when the player is still not on fire.