When receiving a notification about a new e-mail message, clicking on that notification opens the e-mail in the app, but doesn't bring the app itself into focus. If the app is behind others, it won't be seen. Other apps must be minimized or the ProtonMail app icon must be clicked in order to bring it into focus to see the e-mail that was opened.
Unofficial desktop client for ProtonMail 1.2.1
Electron 8.5.2
darwin x64 19.3.0