the Vrock in Book 2:
According to the rules, a creature must spend one action every turn to Fly or they'll fall on the ground: "If you’re airborne at the end of your turn and didn’t use a Fly action this round, you fall.". (https://2e.aonprd.com/Actions.aspx?ID=94)
However, the Vrock's Dance of Ruin (https://2e.aonprd.com/Monsters.aspx?ID=100) takes three actions every turn, which prevents the creature from using an action to Fly. Unfortunately, there are no mentions, in the ability, that it compensates somehow for the missing Fly action, except to say that it "dances in flight".
I imagine this could be a recurring issue with any flying creature that have 3 actions abilities (even more so casters that are flying), but potentially also a similar problem with swimming creatures.
Is that a voluntary situation, or something no one thought about?
Shouldn't staying 'steady' (same square/cube) be at least a free action? This would fix all problems.