When you build walls on top of and besides each other in 1.15, they all get the <direction>=true state, in 1.16, the top layer gets <direction>=short and the lower layers get <direction>=tall, this is correct.
But when upgrading from 1.15 to new versions, the context of the direction states isn't correctly checked, and just maps false=none and true=short. A block update will correct the walls.
This is also true for the post of walls.
Walls generally don't have posts (up=true), when there is another block the wall connects to ontop of it, if there is also a block they connect to on opposite sides.