When jumping or going down stairs, you briefly land 1 block below where your meant to

Confirmed. Reproduce by setting down a singular block. Place a line of blocks one block higher, with a two-block gap. Sprint over the line of blocks, toward the singular block. The player falls across the two-block gap onto the singular block, but the landing is a bit...rubber-bandy. For a moment, it seems as though the player actually lands in front of the singular block, rather than on top.

I should add, what I think is probably happening is that, while falling, the player is hitting the top half of the side of the singular block. Then, the player can "walk" up the less-than-half-block height, in the same way a player can walk up a sequence of half slabs. In other words, the horizontal velocity seems to be taking precedence over the vertical velocity.

Can confirm sinking into the ground; I think it is clientside only but unsure

This may or may not be related, but I experience what is described here when moving about while wearing Elytra.

Then this must be MC-90591.

It is not a duplicate, that above comment was just confused as to which bug this was. Elytra are not mentioned anywhere by the reporter.

Admittedly i am wearing Elytra and had completely forgot i had it on, so it is actually related as this dosent occur when not wearing it.