After 10.15.4 update Guest User popped up on Login screen with unknown password.

Yes, intentionally, or not, major macOS updates, or reinstalls, bring back the Guest User account, even when it was disabled.

As you noted however, the default Guest User account doesn’t use a password. It doesn’t save documents or data, so there’s no need for a password.

After an update to macOS Catalina 10.15.4 (19E266) from Mar 24, 2020 (made with downloaded Combo Update), among other user accounts the Guest User popped up on my Login screen.

More interestingly it has some password!? Which I don't know. Guest User is not supposed to have a password, right!?

I suppose you can use any administrator account to reset the guest account's password, but I wonder whether it will work as the password is supposed to be blank anyway!