I have more than one protonmail account (some for voluntary work and a private one for myself).
When I go to protonmail.com, then I'm automatically logged in to the first account (it seems they're sorted alphabetically).
I then switch account, read / write some mails, and close the window.
Later, when I come back, I am again logged into the first account, not the one I used last.
It would be more convenient if I was logged into the last used account.
Bonus feature: Some users might want to always be logged into a default account. You could make it an option to be logged into a certain account always or the last used account.

Off-topic: It's way too difficult to give feedback.
1: You enter the subject, thinking that this is the entire message, and you don't want to click the button to post a new topic, because it's not your entire message.
2: When you're about finished writing your message, you find out that you need to register an account to post - you can't use your Protonmail login; eg. you need to create a new login with different password rules, so you'll want to save the message you've written (copy-and-paste), before you register
3: After registering, you need to sign in again. This could have been done automatically when you've confirmed your email address.