Freedom of Movement doesn't prevent grapples or the Grappled condition. You can spend 5 movement to end the grapple. Can you "spend" this movement to get out of the grapple immediately (i.e. on the grappling opponent's turn)?
Setup: A Kraken has a multiattack allowing it to make 3 tentacle attacks or to substitute its Fling ability for any tentacle attack. When a tentacle attack hits the creature being attacked is automatically Grappled. A Fling action allows a held or Grappled creature to be thrown 60 ft, causing damage and a prone condition.
Scenario: A Kraken can use its Multi-attack to attack a creature with Freedom of Movement on it, cause the Grappled condition, then with it's next action Fling the Grappled creature.
Can the creature with FoM prevent this Fling during the Kraken's turn by spending 5 of its next turn's (or last turn's "saved") movement to escape the grapple?
RAW it seems that the Kraken would be able to do anything it wanted to a FoM enhanced individual as it would any other. In fact, if using Legendary Actions it could Grapple after the FoM character's turn and then on its own turn Bite (and thus Swallow Whole) the FoM character. Does this seem to be the intention? Or should the Kraken not be able to grab and eat the FoM character (assuming they have movement to spend)?
Edit: Removed my talk about saving movement or not being able to spend movement you don't have. Per SageAdvice (https://www.sageadvice.eu/2017/12/15/is-freedom-of-movement-broken/) you can spend movement when your movement is 0. So the issue isn't if you can spend movement (that you may not have) but if you can do it on an opponents turn (to prevent further bad effects that can result from being Grappled).