Under the precedent set by existing climbable blocks (e.g. vines, ladders) as well as flight in creative and spectator modes, the established control scheme is: jump = go up, sneak = go down (or go down faster), jump+sneak = don't go up or down.
Scaffolding as implemented in 19w04b does not have this behavior. Instead, the jump key overrides the sneak key so that holding both makes the player climb up. To be consistent with the above, jumping and sneaking should cancel each other out when both are held at once.
Linked issues
is duplicated by 3
relates to 2
Comments 13

Affects 1.16 Pre-3
Still happens in 1.16.4
Can confirm in 20w48a.
Can confirm in 21w03a.

Can confirm in 1.17.
Can confirm in 1.18.
Can confirm in 1.18.1.
Can confirm in 1.18.2 and 22w11a.

Can confirm in 1.19.4
This current behavior makes it also pretty difficult to place scaffolding where you are looking (requires sneaking) instead of using the "assisted" placing.
Fixing this bug would make that easier.