The bug

Scoreboards with minecraft.custom:minecraft.jump don’t increase when jumping in minecraft:generic.scale*1.5 blocks.
This might be intended, because the y level can’t change but I am not sure. You are actually able to jump in minecraft:generic.scale*1.5 blocks high spaces, but you don't really notice it when pressing space. You are however midair for a short time when pressing space, which means, that you are able to jump, but the scoreboard doesn’t detect it.

To reproduce

1. Create a scoreboard with minecraft.custom:minecraft.jump
2. Set it to your scoreboard to the sidebar
3. Create a minecraft:generic.scale*1.5 blocks high area using slabs
4. Go into it and jump
   → The scoreboard won’t increase

You can also put these two commands into two repeating command blocks to know, whether you are considered to be on the ground or not:

1. title @a[nbt={OnGround:1b}] actionbar {"text":"On Ground"}

2. title @a[nbt={OnGround:0b}] actionbar {"text":"Midair"}