I have had it a few times too, both red and green. This bug has been reported to together.jolla.com, at least twice:

https://together.jolla.com/question/...-phone-yammed/

and

https://together.jolla.com/question/...ly-shuts-down/


In the second thread there was a message someone had gotten from Jolla care:

"We are sorry to hear about your continued problems with your Jolla phone. We have the following two options to offer:

1) Wait till the next SailfishOS update - planned to week 10. It will contain a fix for the phone-turn-off-by-itself issue.

2) Send your phone to service (we would tell you how).

Please let us know which option you want to take.

Sincerely, Jolla Customer Care"

So hopefully it will be taken care of in a few weeks.