"The target can move only by pushing or pulling against a fixed surface, which allows it to move as if it were climbing."

I feel that this is being hyper-focused on; while this creates an interpretation dilemma, what's really more important here is the other interactive part of the spell, and how it functions in each case of interpretation.

"You can change the target's altitude by up to 20 feet in either direction on Your Turn. If you are the target, you can move up or down as part of your move. Otherwise, you can use your Action to move the target, which must remain within the spell's range."

Interpretation 1: Magic is magic and the flying creature needs a SOLID surface to move around on or it hangs there indefinitely, unless you use your action to move it on your round, straight up or down only, by up to 20 feet. It's stuck. Personally, I feel this is obviously not the intended metric, otherwise earthbind would be absolute garbage when held up next to it. The spell also doesn't use the word 'solid.'

Interpretation 2: Magic and physics can coexist. The text doesn't actually say you need a 'solid' surface, it says you need a 'fixed' one. By flapping your wings, you are 'pushing' off the fixed surface of the ground by displacing the air between you and it.

This works with Mearls' answer, and the way a round would play out in this use-case follows.

Round 1: Griffin takes to the air and begins its approach for a flyby attack. You cast levitate on the griffin- it fails its save. You immediately change its altitude by 20 feet straight up or down.

Round 2: The griffin can still use its flying speed to move and take actions. It does so, making a pass at you and missing. It flies past you, and circles back around to make another pass. Here you have options on your turn. You can use your action to move it up or down another 20 feet, you can ignore it and make an attack on it, or you can Ready an Action with the trigger "when the griffin flies within five feet of someone, lower it up to 20 feet down to the ground." How this plays out in RP is entirely up to your DM in terms of what consequences or damage the griffin suffers from this, if any, but there's one mechanical consequence to this RAW that can't be overlooked- if you force the creature down to the ground before it attacks the target, it's no longer flying when it attacks the target, and it will no longer benefit from flyby, forcing it to stay and engage or take an AoO to resume moving away.

You could also raise it 20 feet, possibly causing it to have to fly back around for another pass to make an attack based on its size, reach, and maneuverability, which could keep it from ever actually being in position to make a flying attack and force it to land and stay on the ground if it wants to make an attack.

"The target can move only by pushing or pulling against a fixed surface, which allows it to move as if it were climbing."

I feel that this is being hyper-focused on; while this creates an interpretation dilemma, what's really more important here is the other interactive part of the spell, and how it functions in each case of interpretation.

"You can change the target's altitude by up to 20 feet in either direction on Your Turn. If you are the target, you can move up or down as part of your move. Otherwise, you can use your Action to move the target, which must remain within the spell's range."

Interpretation 1: Magic is magic and the flying creature needs a SOLID surface to move around on or it hangs there indefinitely, unless you use your action to move it on your round, straight up or down only, by up to 20 feet. It's stuck. Personally, I feel this is obviously not the intended metric, otherwise earthbind would be absolute garbage when held up next to it. The spell also doesn't use the word 'solid.'

Interpretation 2: Magic and physics can coexist. The text doesn't actually say you need a 'solid' surface, it says you need a 'fixed' one. By flapping your wings, you are 'pushing' off the fixed surface of the ground by displacing the air between you and it.

This works with Mearls' answer, and the way a round would play out in this use-case follows.

Round 1: Griffin takes to the air and begins its approach for a flyby attack. You cast levitate on the griffin- it fails its save. You immediately change its altitude by 20 feet straight up or down.

Round 2: The griffin can still use its flying speed to move and take actions. It does so, making a pass at you and missing. It flies past you, and circles back around to make another pass. Here you have options on your turn. You can use your action to move it up or down another 20 feet, you can ignore it and make an attack on it, or you can Ready an Action with the trigger "when the griffin flies within five feet of someone, lower it up to 20 feet down to the ground." How this plays out in RP is entirely up to your DM in terms of what consequences or damage the griffin suffers from this, if any, but there's one mechanical consequence to this RAW that can't be overlooked- if you force the creature down to the ground before it attacks the target, it's no longer flying when it attacks the target, and it will no longer benefit from flyby, forcing it to stay and engage or take an AoO to resume moving away.

You could also raise it 20 feet, possibly causing it to have to fly back around for another pass to make an attack based on its size, reach, and maneuverability, which could keep it from ever actually being in position to make a flying attack and force it to land and stay on the ground if it wants to make one.