Venu 3s total ascent for activities has gone mad - recording way too much

For the past three outdoor activities I have done the past three days (walk, hike, run), my watch has recorded WAY too much total ascent. 

I walk and run the same course every time, so I know what the ascent should be, it's pretty small, less than 200ft.  Today, it recorded almost 1500. 

For a hike I did yesterday, which was a gentle social ramble, my watch recorded over 10k ft gain, when in reality it was about 500. 

The chart for an average run or walk elevation gain (same course):

Todays's run:

The walk from 3 days ago:

For the hike I did, I hiked with a friend who has a Venu 2, and their info came out correct:

While mine looked like this:

I restarted my watch after the hike fiasco yesterday, and it still recorded elevation/ascent incorrectly for my run today.

My watch gets cleaned regularly, gets a gentle wash with hand soap every few days or so and leads a pretty quiet life when it isn't tracking an activity (like I can't think of what could have happened to damage any sensors?)

Anyone else see this kind of weirdness before?  Solutions?

Edit: 

If I'd googled it first, I would have seen that there is a way to clean and re-calibrate the sensor, so I did that and will try it out on a walk later & report back.

Edit 2:

After cleaning and recalibrating, I had a chance to go for a walk on my usual course today.  It was wonky for the first 2.5 miles or so, but then seemed to return to tracking as accurately as it ever had in the past.  This mimics the "3 days ago" walk from above.   I'll be doing the same course again tomorrow, let's see what happens then.