Rules as written: Yeah, this seems to be the case.
The rules for High Jumps state:
When you make a high jump, you leap into the air a number of feet equal to 3 + your Strength modifier (minimum of 0 feet) if you move at least 10 feet on foot immediately before the jump. When you make a standing high jump, you can jump only half that distance.
When calculating your standing jump height, you have to factor in the "Round Down" rule from the introduction to the Player's Handbook:
Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.
So with a strength of 14, you have a modifier of +2, your standing jump height is 2 feet, so a 2 foot fall from the apex of your jump translates to 4 feet of lateral movement. Does this lateral movement also expend available movement? No. This feature is modifying how you fall, and since falling does not usually expend available movement, neither does falling in this particular way, since it does not specify that this particular change to falling now costs movement.
This works just fine if you are measuring distance to the foot, but it is unclear how this functions if you are playing on a fixed 5-foot grid. In this case, you will have to ask your DM.
Rules as intended: This makes for a significantly overpowered feature, and the narrative image is ridiculous.
This, to me, seems to be a bug in the feature. If we take the feature to mean "double all your movement for free", then it is absolutely broken. It goes from being a good feature in a specific context, to being a blanket improvement in any context where movement is measured.
Further, it is even more powerful than if a feature just said "Your speed is doubled". A racial feature that said "your speed is doubled" would already be extremely powerful, but this feature is essentially that, with the fall protection.
Further, as written, it does prevent opportunity attacks: the part of the move where you leave the creature's reach isn't actually using your movement, which is sort of a cherry on top of an already broken feature. This simply cannot be the proper application of the feature, because that would make it far and away the most powerful feature in the game. Compare it to the Mobile feat:
- Your speed increases by 10 feet.
- When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn.
- When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not.
Mobile is generally regarded as a very good feat. Allowing Manta Glide to work as you have described makes it a vastly superior version of the Mobile feat.
Mobile increases your speed by 10 feet, Manta Glide doubles it. Mobile lets you cancel difficult terrain if you take the Dash action, Manta Glide lets you do it for free. Mobile lets you prevent opportunity attacks from a creature you attack, Manta Glide lets you prevent opportunity attacks from any creature for free.
There is no way this is the correct interpretation. Manta Glide should simply not be allowed to function this way.
Also, the mental image of a character jumping a bit then gliding a few feet as their primary locomotion pattern seems quite silly to me, but that's mostly an opinion of mine. Maybe you think that's cool. That's okay. The primary issue here is that this reading of the feature is so overwhelmingly powerful that it cannot be correct.
Making Manta Glide cost 1 Reaction or limiting its use to "once per turn" mostly resolves these issues.
Darth Pseudonym pointed out in a comment:
So what I'm hearing is it could all be fixed by making Manta Glide a reaction.
This is exactly right. Giving it a reaction cost limits its use to once per turn, or if you want to do so without giving it a cost, just make it usable only once per turn. It still functions as intended, but can no longer be abused as described in the question.
And finally, as Mark Wells observed in a comment, this change would make the feature consistent with similar features that affect the rate of falling such as the spell feather fall which has a casting time of 1 Reaction, and the Monk's Slow Fall feature:
Beginning at 4th level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your monk level.
I've gotten lots of helpful observations from users in the comments; this time Ryan C. Thompson pointed out that this may not resolve the "free disengage" issue. At a strength of 14, giving a 4 foot glide, this is not enough to leave a creature's 5 foot reach, the last foot to leave their reach will use movement and provoke an opportunity attack. However, at 16 strength, the glide is increased to 6 feet, which reintroduces the issue, but only if you have ruled that it is once per turn, but not if you have ruled that it costs a reaction. The rules for opportunity attacks state:
You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.