Jan, Morten, thank you for your replies! I've been running some tests, flew up to FL370, down to 350, then up again to 370, down again to 270. Speed was mach .80.
The first time the vertical speed went to 6000 ft/min and stayed there until it reached 350. Speed went up to mach .82.
The second time was worse, this time I enabled the data output window (see screenshot). It was at 7600 ft/min for maybe 6-7 seconds then stayed in the 6000s until I told it to go to 370 again.
EDIT: I did three more tests, same altitudes, two times starting at .80 mach and the last time starting at .75 mach. This time I let the aircraft reach the selected altitude, all three times the result was the same. At first the descent rate goes to 7500+ for a few seconds, then 6000+ and finally it manages the descent properly, keeping the dialed in speed. When starting at .80, it will momentarily accelerate to .82, when starting at .75, the speed raises momentarily to .78 mach. Hope this helps.