After last update my watch Instinct Solar is shooting down when I want to perform some activities? Why?
It's happening in the same way to my watch as you explained above. I strongly belive it's a bug that Garmin team has to fix this immediately with a new update.
I have already received my replacement watch. Garmin support was really fast.
After the update to version 17 everything seems to work normally. I'm still testing though.
I have same issue. It started few days ago on walking activity, it turned out that watch shut down at every 1km point when alert sounded. Then I noticed it on running activity, also on lap alert. Then…
Go to the Garmin Support Center. The phone number will be listed for your country.

Worked for me. Thanks!
I disabled all vibration alerts also, because they shuts down the watch when inteligent notification were received, and GPS related functions. But pulsoximeter continued shuting down before endind measure, It was solved deleting folders and .bak files as mentioned.