This happened last night in our session and we couldn't find a conclusive answer:
An NPC is grappling a polymorphed PC in the air (the PC polymorphed into a Quetzalcoatlus, and we didn't at the time realize the NPC couldn't have grappled the PC due to size differences), and through some trickery the NPC has haste cast on it by a hostile PC, accepts it, and then the hostile PC immediately breaks concentration to cause the negative effects of haste on the NPC.
What happens to the grapple initiated by the NPC? Does the grapple end when haste ends (the NPC can end a grapple instantly), or does the PC have to still actively break out of it? Does the grappled PC stop falling when the haste spell ends on on their turn if they break out of the grapple? Can the NPC maintain the grapple if the PC doesn't even try to break out (for whatever reason)?
The language around the haste ending effect is kind of ambiguous and doesn't use the keywords you'd expect, and I'm wondering if that's on purpose, or what that means.