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 1.16, the context of the direction states isn't correctly checked, and just maps false=none and true=short. The wall we build in 1.15 doesn't become the same wall build in 1.16. A block update will correct the wall.