I have had two flights where I was in level flight on AP when my A321 developed a progressively more severe pitch oscillation. The first one put me into a stall at FL28. Fortunately I caught it in time to avoid a bunch of violations on the expert server. I figured I left some aspect of the AP off and chalked it up to pilot error.
The second flight immediately after also exhibited the behavior at FL24 although I caught the oscillation much sooner.
I am now in level flight at 16000 and am not experiencing oscillation. Winds are calmer too although I don’t know if that was a factor on the first flight or at all.
Is this a known issue? Would you like more repro steps?