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When installing an APK, it automatically extracts the libraries of the right arch type and has them available for System.loadLibrary. But, if you're trying to load a non-standard library, the installer won't know your app has 32bit native code.
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java.lang.UnsatisfiedLinkError: dlopen failed: "/data/data/com.test.mapp/app_files/.???" has bad ELF magic