Height limit for building is <limit> blocks, when failing to place the top half of a slab at the top of the world

I was standing on the lower half of a slab and accidentally tried to place the upper slab directly under me, causing the block to fail to be placed, however, it gave a message saying "Height limit for building is 255 blocks"

It shouldn't have shown this message since where I tried to place the block was still in the height limit

(This was in the nether so the height limit wasn't 319)