In their own policy:
> “In addition to the items listed in our privacy policy, in extreme criminal cases, ProtonMail may also be obligated to monitor the IP addresses which are being used to access the ProtonMail accounts which are engaged in criminal activities.”
So there's no question whether or not they do it, it's more of how often they do it and for what. The French case was a big deal because it didn't seem to meet the "extreme criminal case" threshold, and yet the logging was still carried out.