See attached images.

You will probably find this easiest to test with husks or similar player-like enemies (Other enemies, like wolves, I found, would occasionally accidentally walk up snow banks they couldn't pathfind up).

Scenario 1 has a stack of snow 5 piles, or 10 pixels, high, giving it a hitbox identical to that of a half-slab (Since the hitbox of a snow pile is always 2 pixels shorter than the height of the pile) at 0.5 blocks high. Mobs can path up and down this easily, just like with half slabs. No bugs here.

Scenario 2 has a stack of snow 6 piles high (0.625 blocks) in the back and one 2 piles high (0.125 blocks) in the front. You can walk up that slope without jumping yourself. Mobs, however, cannot pathfind up that slope. They will move to stand on the shorter pile, and then stop. They won't even try to jump up the slope. That is a bug.

Scenario 3 has a single stack of snow 2 piles high (0.125 blocks). You can't walk up that slope, but you can jump up. Mobs also cannot pathfind up this. They will just move to stand on the snow and stop. They will not attempt to jump. That is also a bug.