I've been working on making pretty jump tests for horses and have discovered that when stacked walls do not seem to be even 1 block high, let alone the 1.5 they're supposed to be on any middle parts of the wall.
A horse that can jump 2.5 blocks, but not 3 blocks when made out of full blocks and slabs or fences can still jump over the middle of walls stacked three high. A donkey, which can only jump 1 9/16th blocks can still jump over walls stacked two high.
Any wall that has the raised pillar part (the end caps, or if for example a string is placed on each middle part) goes back to being taller.
From my testing, it seems that a stacked wall is without a pillar is half a block high and causes the wall beneath it to be 1 block high, therefore three stacked walls are 2.5bh in the middle and 3.5bh on the ends. If I add a string on top, to turn each middle part into a pillar, the whole wall again becomes 3.5 blocks high as expected.
I haven't been able to find any information about this being intended or even known.
Attached a screen shot that hopefully demonstrates the issue clearly, everything in the image is the equivalent jump height, except for the gray parts of the wall.
This is because the posts inside of the have a slightly larger hitbox than wall pieces that don't have a post. As a player, you can stand on that slightly larger hitbox:
[media]This has been "fixed" in 20w06a since walls no longer cause other walls below them to create a wall post (by default; in my screenshot I have forced the posts to appear by placing more walls behind the wall).