OK, I got your point now.
It looks to me this is your android client’s problem. Is it the latest version?
I just conducted a test on an Android device (fresh install version 2.2.1 from Google play store), trying to open an encrypted library that is never opened on this device before and not opened in the web-interface in past 1 hour. It DOES ask me for password.
Note that without a password, the file tree of an encrypted library may still be browsed because it is not encrypted (https://manual.seafile.com/security/security_features.html). However, the content cannot be downloaded. So I suspect what happens here is the android client fail to detect this is an encrypted lib, open it as a normal lib, show the file tree, but fail to download the file content.