I couldn't find the code on how switching gamemode does this, but I'm using player.capabilities.allowFlying to give flight, however when the duration of the item runs out, the player doesn't drop if they're flying when it happens, so how do you do force them out of flight?

Player.capabilities.isFlying = false

That's what I already tried, it just leaves you floating until you double tap spacebar again.

you can do this

player.capabilities.allowFlying = false;

this will make the player drop down but this will also cause a conflict with other mod's flying abilities.

That doesn't work either.

 

Edit:  It turns out it was something unrelated getting in the way.  I'll have to use a combination of both of those though.