I found a bug last night too here's how you replicate it:

1. Get a group together with someone who can teleport in it.
2. Start playing the levitate song.
3. Ask the TPer to cast group teleport.
4. Observe as the spell does not take hold.
5. Stop singing the levitate song.
6. Have the TPer cast group teleport again.
7. Fasten your seatbelt.

Conclusion:
Group teleport will not take hold if a bard in the group is singing Aria of
Eagles.

-Faras