Floors climbed go over 50 per day...

This may be a result of the altimeter being calibrated during a walk. Depending on GPS accuracy the calibration is often way off. This should not happen since the software should recognize this.
I would try to disable Auto calibration of the altimeter and observe for a while.

I went on a horseback ride and the watch showed I climbed 14 flights of stairs and I never left the saddle! Hahahahah

Floors are counted by a change in air pressure, (and there seems to be a time and steps component) so even if you are on a horse, if it goes you a hill, the air pressure changes, and you can get floors. Are you also getting steps?

I noticed today on a hike that I got stairs counted on steeper hills but not on longer slopes, even if I gain more than 3m of altitude.
My guess is that, additionally to the altitude component (3m per flight of stairs) there is the motion sensor involved to 1) recognize if you are moving forward at all (no elevator count) and 2) limit the steps that can be taken on 3m of altitude to count as a flight of stairs.

Mine was counting while having dinner...it even went over my goal (15 stairs climb)...

While having dinner, did the HVAC click on/off, as that will change the air pressure, and while eating, the watch on your wrist will see movement! Are you sitting by the fridge, where each time it's opened, cold dense air rolls out?

Sometimes, it's just not understanding what you see....

The barometric altimeter on many VA3 units (including mine) simply stopped working properly at some point in fw update history.
Although absolute altitude measurements depend on calibration, checking my activities history in garmin connect I'd say there is a correlation between the update to fw 2.80 in late December last year and the barometer starting measuring nonsense altitude variations.
As a consequence, floor count became totally nonsense since then.
See for example:
https://forums.garmin.com/forum/into-sports/health-fitness/vivoactive-3-aa/1319992-altitude-is-very-incorrect
and many other similar discussions about worng elevation and floor count.

While having dinner, did the HVAC click on/off, as that will change the air pressure, and while eating, the watch on your wrist will see movement! Are you sitting by the fridge, where each time it's opened, cold dense air rolls out?

Sometimes, it's just not understanding what you see....

Having dinner inside the house with windows open HVAC off just ceiling fan on.

If you were in and out of the draft, that could explain it. Hard to say....

I think these devices are pretty much useless when it comes to counting floors. I never had an issue with my 935. I downloaded the latest beta earlier Saturday morning. Floors were correct that day. Yesterday, I was in the house watching TV. No AC, the fridge wasn't opened, no tornadoes, hurricanes or gale force winds passed through my living room. 12 floors! The barometer was steady. The beta right? Maybe not. Today at work I did 5 flight of stairs and my watch is showing 4. I think I'm going to delete the widget instead of trying to make sense of any of it.