Certain countries (i.e. Russia) restrict access to Proton Mail. Apparently, Pass uses some of the domains or IP ranges they block. And if Mail access can be circumvented with a VPN, being unable to access your passwords is much more serious as you might as well have your VPN credentials saved there. In addition, connecting to VPN is most inconvenient whenever you need to unlock your vault or save a new item. This could be solved by having dedicated IPs and domains for Pass backend, so it does not suffer collateral damage from other blocks.