Why implement Fly at all if it's just another Jump?
My guess would be some kind of limitation of their pathfinding algorithm, it is quite a lot of work to make it work for not a lot of payoff in the case of BG3.
They could have saved themselves the embarrassment and not implemented Fly at all. But they chose to underline a glaring weakness of their AAA engine. While in Solasta you can actually fly and climb walls.
Solasta is grid based, which makes it infinitely easier to implement a fly spell for. Pathfinding for grid based system is pretty much a solved problem thanks to algorithms like A*.
