Apps will not auto start on Android 9.0 Pie
This would not be a complete answer to your problem.
But if you suspect that BOOT_COMPLETED broadcast is disabled for some reason, you can try to send it manually using adb shell and see if your app starts up.

It better to send the broadcast directly to your app by specifying its package name. If you do not know the package name of your application, first you need to find it. To get a list of all installed packages in your device, you can use following command.

$ adb shell pm list packages


After finding the package name of your app from the output of above command, you can use following command to send the broadcast to your app, and see if it starts after it received the broadcast.

$ adb shell am broadcast -a android.intent.action.BOOT_COMPLETED -p &lt;com.your.apps.package.name.comes.here&gt;


In above command replace &lt;com.your.apps.package.name.comes.here&gt; with your app's package name.