The bug
The top of sandstone wall and red sandstone wall are the side texture instead of the top texture. This has nothing to do with the new textures, it's just that the side texture is applied to all faces, which is a mistake.
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Affects 18w47b

Affects 18w48a

Affects 18w49a

Affects 18w50a

Still affecting 19w13a/b and probably 19w12b, if I remember correctly.

Affects 19w14a

Affects 19w14b

Confirmed for 1.14 pre-release 1
Still an issue both in 1.14.4 and 1.15 pre2

Confirmed for 20w06a.
There are two possible fixes for this: one is really easy and not really a fix, which is using the bottom texture instead of the side texture. The other one requires creating "template" models for walls which support 3 different textures and then using those templates for sandstone and red sandstone wall models. These fixed models are included in the attached resource pack.
[media]
Affects 20w11a
It also affects blackstone walls, the side texture is used in every face
Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05a.
Can confirm in 21w06a.
Can confirm in 21w07a.

Affects 1.17.1

Affects 1.18

Affects 1.18.1

Affects 1.18.2
Affects 1.19 pre-release 1

Affects 1.19
Can confirm in 1.19.2.

Can confirm in 23w07a.

Seems fixed in 23w14a

I can still reproduce this in 1.20.1. @Alex perhaps you're using a resource pack?
[media]

Can confirm in 1.20.5 pre4.

Can confirm in 1.20.5.
Affecting 18w46a