When using file browser, do you select ‘open’, or ‘install’?
BTW- “installing” an apk in Android just means moving the apk to /data/app, which yes could do manually…
There’s also apk installer apps, and apkd install from Android terminal, etc… (But, I must admit I’ve been out of touch with Aliendalvik since a few updates ago, so maybe some issue I’m not aware of…)