I'm using Magic Leap 2, OS 1.3.0-dev.
I'm trying to install my own application by downloading the APK from our web-server using Magic Leap's build in web browser.
Download the APK is working. Also granted the install permission. However, an error message shows up telling that the APK was not installed.
By any chance do you have the "testOnly" attribute set on the application tag in your Android app manifest?
If that attribute is set, then the apk will only be installable using adb. You can pass the -t parameter to the adb install command to install a testOnly application. ("adb install -t yourapp.apk") Alternatively, you can also use ML hub (or the play button in Android Studio) to install applications with the testOnly flag. ML hub will ultimately shell out to adb, so using it to deploy and install works as well.
While it may be a little strange, this is expected behavior on Android. You can see the official documentation here - <application> | Android Developers
Hope that helps! Please reply back if that wasn't the issue.