I discovered this using the Light Drifter spell from Roots, which temporarily sets a player into Spectator mode. Basically, a player in spectator mode has creative flight active which the flugel tiara counts as being in use.

This also occurs when the player is in creative mode and takes flight.

Version Information

Minecraft: 1.12.2
Forge version: 14.23.5.2838
Botania version: r1.10-361

Quick update: the tiara still depletes mana when flying in either mode, even though the flight bar will not go down now.