The bug

When an entity travels fast on #climbable blocks, the x/z-component of the entity's delta movement is clamped between -0.15 and 0.15 causing the movement vector to be 45° with respect to the x/z axis.

How to reproduce

1. Add minecraft:oak_button to #minecraft:climbable.

2. /fill 0 0 0 15 0 15 minecraft:stone

3. /fill 0 1 0 15 1 15 minecraft:oak_button[face=floor]

4. /effect give @s minecraft:speed 1000000 255

5. /tp @s 0 1 0 -1 90

6. Walk forwards
   → The player walks southeast.

minecraft/tags/blocks/climbable.json

{
    "replace": false,
    "values": [
        "minecraft:oak_button"
    ]
}