Floors counted was fine until 15th of August which is when software update 4.90 was released and installed.
I notice that Sensor Hub was also updated at same time to 20.04, so I suspect that this is real culprit.
SEE: vivomove 3 Series Software Update - 4.90 - vivomove 3 Series - Health & Wellness - Garmin Forums
My symptoms are as follows:
1) Everything working well until 15th of August.
2) No floors recorded 15th of August to 14th of September. Exactly one month. Maybe a clue.
3) Bizarrely, from the 15th of September it has been counting very high number of floors. Possibly 4 x actual number of floors.
I believe it is too much of a coincidence that problems started after software update, and in particular Sensor Hub update.
Hopefully next update will correct this error.
If you need more info, please ask.