Oh, I thought it was just me! All the Airbus planes have horrible autopilots that gradually rise and fall, in a barely noticeable way. I found myself feeling irritated and wished the source code was opensource so I could have a look at it.
That’s intentional, it allows the plane to do minor speed adjustments without altering engine speed that much by wandering a bit above and below the set altitude. The autopilot mode is called ALT CRZ (as opposed to just ALT).
Are you telling us this as a pilot or a coder? Calling it's autopilot is horrible is a lot of stretch.
Horrible may be a bit hyperbolic, but it underperforms other airliners.
I don't do well on flights, usually feeling slightly nauseous the whole time, and this made it worse. I could tell the difference from the Boeing jets.