There is a closed issue on joplin github page about this problem.
HTTP is not permitted on Android 9
In this issue laurent22 say “We already set usesCleartextTraffic but looks like it makes no difference.” but it is not true, usesCleartextTraffic is working.
I checked network_security_config.xml in apk, there are only few domains allowed such as:
I have tried to add new line to it with apktool, but i can not have the modified apk installed
Then I installed portforwarder[https://play.google.com/store/apps/details?id=com.elixsr.portforwarder&hl=en_US], and setup joplin to use localhost instead of the server real ip, it works.