I'm building a map and I'm using a scoreboard objective do determine how long a player is TPed vertically (Z/-Z) and these are the commands I've used:

execute @a ~ ~ ~ detect ~ ~-1 ~ quartz_block 0 execute @p ~ ~ ~ detect ~ ~-2 ~ redstone_block 0 scoreboard players set @p Cannon X

scoreboard players remove @a[score_Cannon_min=1] Cannon 1

tp @a[score_Cannon_min=1] ~ ~1.7 ~

Cannon is a 'dummy' objective and X is because I've tried mutliple scores to determine distance AND when the bug happens.

I'm not sure why but when the player hits something and starts falling, they'll fall to the ground, and suddenly they'll get returned to where they were they bumped into something, in this case the roof. If the score is set high enough to go over the roof it'll land and tp once again after landing. Bouncing on the block like a trampoline and landing outside of it will cause the bug to happen too. The only time when it does work correctly is when the score is so low that you basically tp up and down on a loop, which is what I'd expect.

So what I'm trying to say is that sometimes /tp does mutliple TPs to the destined location, be it specific or relative coordinates.