This is quite complicated, and I don't know exactly what is wrong, but I know something is wrong.

In this first screenshot, I have 8 2x2s of walls. The 4 2x2s on the left are the new deepslate walls, added in 1.17. On the right, there are 4 2x2s of arbitrary walls, the only difference about them is that they were added before 1.17. This is with no texture pack, just vanilla textures on vanilla, un-modded 1.17.

In this second screenshot, is the same setup, but I have a resource pack loaded. In this resource pack I have changed a lot of the block models, but specifically for wall models I:

1. Left models/block/template_wall_post.json unchanged
2. Edited models/block/template_wall_side.json and models/block/template_wall_side_tall.json so that they are invisible; removed all cubes.

Clearly something is wrong, because with the walls on the right (the old walls), you can see all of the individual posts, but with the walls on the left (the newly added ones), there is nothing there except for the posts on the ends.

This is odd for 2 reasons:

1. Obviously because you would expect that all the walls look the same, no matter when they were added to the game.
2. In the photo with the vanilla resource pack, you can only see the posts on the ends of both sets of walls, but in the one with the resource pack you can see a post on each block of the right walls

This makes me think that the newly added walls are rendered differently than the old ones. Or something.