If you place any wall block (I've tested them all) between 2 blocks along the X-axis, the 'up' tag will be set to on, whereas if you were to rotate this setup 90 degrees (in between 2 blocks in the Z-axis) it will not be.

After updating the block with 'up' tag by placing and removing a block next to it it gets reverted to being without 'up' tag.

I hope this explains it, if not watch the video I provided