This is an issue I have since day one in all planes (even the pmdg dc6). It doesn’t matter what your baro setting is, the plane will level of to where the altitude would be if baro was still 29.92.
Example: DC6, baro 29.92, activate altitude hold on 4000ft. Keeps 4000ft. Baro 30.00, activate altitude hold on 4000ft, plane decends to ± 3970ft.