Open in ProtonMail doesn't login with Vivaldi

I've stored the session permanently and auto checking seems to detect new e-mails, but when I click "Open in ProtonMail" button, it shows a ProtonMail login page instead of just opening the inbox. Any idea why this only seems to be happening in Vivaldi?

Windows 10 2004 64bit
Vivaldi 3.0.1874.38
Latest ProtonMail (Unofficial) from Chrome web store.