FD Pitch Bar Drifts Down in Altitude Hold

[tucsonphil]

With FD ON in Altitude Hold and AP OFF, the pitch bar behaves normally. When I engage the AP the pitch bar slowly drifts down. See photo. Have not seen this before and can't figure it out. The AP will hold altitude just fine. If I disconnect the AP, the elevator is in trim.

Yes I did see the traffic coming at me at the same altitude.

Any ideas?

Phil in Tucson
RV-10
HDX