Steps to Reproduce:

①Create a new world, build a two-block-high space, as in Picture 1 in the attachments.

②Stand into the space, as in Picture 2.

③Open the shulker box below you. 

Observed Result:

You sink into that.

Expected Result:

An opened box is 0.5 blocks taller than the one isn't opened. So your space for you becomes (2-0.5) = 1.5 blocks. But since there is Short Sneak, you should be forced to sneak, instead of sinking into that.