When looking at the V/S values derived from the FR24 data we will need to take into account that this data is not recorded in a way that allows to have accurate data to derive a precise sink rate from the recorded Mode-S data.
While I agree the reported figures are subject to variation, in my experience, Mode-S V/S data varies not by the thousands, but hundreds in a steady climb or descent.
The track on the other hand clearly shows that the plane was on A/P.
Possibly, considering the difficulties our guys apparently had in maintaining a steady ROD as per LR's suggestion:
Yes, and with very distressed flight-deck crew continuing on what had looked like a reasonable forced landing site. Okay to keep descending.
Could have been the MCP mode they were in, VNAV or even HDG if the winds were constant through the descending layers. I know these are B terms, apologies if I'm not up on my Airbus autoflight vernacular.
I guess we can all make mistakes, but from an aircraft maintenance point of view, I've been told regularly that Lufthansa Technik is top notch.
Is it possible that, for some reason, they didn't know they were descending?
I seriously doubt it. Too many cues to miss. Aside from the instruments - at very least they should have had the standby horizon and altimeter - there is an obvious attitude change, pressure changes along the way, and sound. Then again, with masks on, it could be missed if they were task saturated.