There are multiple posts about this, some dating back from the betas.
Here’s one created by yours truly: Units still don't follow terrain contours and are always fully horizontal
It’s not only siege. All units always remain completely horizontal when faced with an incline. Same thing happens with mounted units and Mongol mobile buildings. You can see horses either clipping the ground or floating.
I suppose the devs have taken some of that feedback and applied it since the Ottoman bombard (which is a new unit) behaves correctly.
The problem seems to be that units in AoE 4 only have 1 contact point on the ground, which is dead center. This works relatively fine for foot units, but for larger things with 4 wheels or 4 legs, it would require 4 separate contacts. It’s a programming costs saving shortcut.