So for anyone interested - this is what I meant 10 or so pages ago.
As depicted above, the stairs look like they are 3D, while in fact they are not. It is impossible to go underneath.
The same was true in the 90's when the DOOM engine had the same limitations, and only the development of Quake brought true 3D - when a point on the bridge and right under the bridge became both accessible.
So in the mounted combat the rider and the mount are located at (roughly) the same point in 2D space, but their elevations are different. What if the game plays the same trick: it looks like the rider and the mount are separate attackers, while in fact they are not? Or, even worse: what if their attacks have different 2D coordinates instead of having the same? So the rider looks together with the mount, while in fact they attack different points on the flat surface?
I think it might lead to the whole bunch of bugs, which are not bugs, but the difference between the expectations from the picture and the limitations of the engine.