Why does my account sign out automatically?

This is a security feature, the idea is to prevent unauthorized access to your wallet. You can set the Auto-Lockout time from the Decentr settings page, by following these instructions.

First, click on the Charon extension in your address bar.

Then click on your account avatar.

Then your account name.

This is the Decentr dashboard. Now, click on the Settings button.

Here you can set the period of inactivity, before the extension requires a password to unlock before usage.