I cannot reproduce this error. It allows me to step up to the full block.
nvm, reproduced, will fix, thanks.
There are a lot of little collision discrepancies like this. For example, Last I checked, If you have a single stack, An empty space above it, And then an upsidedown stack above that, You cannot walk up even though there is more than enough space.
In general, If there is a block with solid collision somewhere in it's bounding box, And it is in the space where a player would walk up another block, You can't walk up the block in 90% of cases.