Actually, I might have some ideas on why this happens.
To be clear still, I’m not programmer nor know any programming/developing things. It’s just my small interest that sometimes I open Android apps’ .apk files and see what’s inside of these. There’s a file called AndroidManifest.xml there which works as the information of your package that declares things like what permission your app use and what components it has etc.
Starting from Android 9, using clear text traffic is denied by default, and the app need to declare a feature flag (not sure what’s that called officially) in the manifest file to reenable clear text traffics (Official docs) and then the app could use http:// links again.
For Obsidian, since user may have internet pictures, I think maybe it is better to add support for this.
Hope this helps @WhiteNoise