D&D is an RPG. The DM puts a structure around the PCs while they improvise their way to glory.
The first rule of improvisation is DON'T SAY NO. If the PC wants to jump across the battlefield, figure out if there is a way to make it work and go with it.
To that end: I'd look to tell a good story. I'd also look to avoid having any abstraction due to the game rules ruin what would make sense to be allowed.
For example: PC Dwarf has strength 20, speed 25', and has had a Jump Spell cast on them. Their maximum long jump, with 10' move prior to the jump, is 60' per the rules, which is more than twice their normal movement.
Rd 1: They attack, move 20', then start to jump a 60' crevice. They cover 5' of the distance. This leaves them in the air, but only slightly off the ground.
Rd 2: They move 50' through the air across the crevice. I would let them use an action during this movement if they liked, even, although it would make it harder to land safely. They'd end the round nearly at the far side, but only a few feet above ground level.
Rd 3: They begin their turn by moving 5' more and finishing their jump. They would still have 20' of movement and an action.
In Rd 1: they use their action to attack and then head towards the crevice. When they reach the edge, they start their huge leap. I REFUSE to let the abstraction of the 'round' prohibit them from doing something that another PC with a faster speed would do within 1 round (a monk with a 20 strength could cover all 60' in a single round), so I would not prevent them from attempting the jump. Between this action and the next they'd be in the air - and if they get hit by an attack, it might influence their trajectory... perhaps making them overshoot or undershoot or veer off target into a wall. I see the arguments that they should be able to jump no farther than twice their movement, but I don't like those arguments and you' still have the problem at the core of this situation - if they jump after using a lot of movement in their round, and you want to allow them to jump up to twice their movement, you still have to figure out how to do it.
In Rd 2: They lept in the air and are going to be up there for 6 seconds during this round. Even though they're continuing to move, they're not needing to do anything to make that movement happen. If they are hit by a spell and rendered unconscious, for example, I would have no reason to halt their momentum. Accordingly, I'll let them use their action as they 'fly', although it will make landing harder. Again, if hit while in the air, it might impact their trajectory. Of course, they can't change direction in their jump once in the air through normal means, but might redirect themselves if things change by casting a spell, using a parachute, or something else.
In Rd 3: Their jump ends and uses up 5' of their movement. If they took an action in the prior round, I'd make them do an athletics check with disadvantage (DC 15) to remain on their feet. After that they'd continue their round as normal.
You might ask, "But what if they try to abuse this to give them the benefits of a levitate spell?" Well, they did have magic cast on them. They are traveling up to 18 feet in the air. They can get some benefits of a higher level spell - as long as it isn't as strong overall. Here, they have some risks involved as well. Anyone remember the Chasm jump in Cabin in the Woods? Let's take away the chasm and have the PC using this to jump across a battlefield and over an army of goblins. Those goblins may move quite a bit while the PC is in the air. Also, the rest of the party may want the dwarf to be in another location by the time he lands. There are some benefits for the PC in the way I'd run it, but also some risks.
Most importantly, it allows me and the player to tell a fun story as his dwarf travels through the air. There would be real tension as the battle changes around him and he'd be stuck in his path. When the Giant comes around the corner, sees the dwarf flying towards him, and readies an attack to 'bat' the dwarf ball back across the gorge... Fun! When the Dragon attacks and hits him with a wing buffet while he is in mid-air? We'd have fun figuring that out. What if he sees the giant popping up after he starts to move and wants to change course in the middle round... what fun things might he come up with to do that?