I stopped receiving #ProtonMail notifications a few days ago on my #Android phone.
I went through the settings, reset my notifications, even uninstalled and reinstalled the app while clearing the cache/data, and still didn't have luck.
Eventually out of desperation I changed my "Private DNS" from `dns.google` to `dns10.quad9.net` and interestingly it started working again 
I'm not saying #Google blocked ProtonMail, but I wouldn't put it past them at this point