Thank you for your feedback! The latest version of the extension asks for your PIN when restarting your browser. Please make sure that you are not using an outdated version. Moreover, in general, you should not log in on untrusted devices, so make sure to log out of Proton Pass after you no longer need it, if your threat model includes such cases.

Please report the issue you mention with the extension dropping the PIN at pass@protonme.zendesk.com, so we can look into this.