freymotif recently google is disabling side loading in Android.
We know Android is open source but google is building it's framework over the Android based.
So if you disabled Google services maximum things in your phone will be stopped working.
So like wise maybe it's an Android Google problem rather than Nothing's.
I don't know much details but Android is nothing without google, and if google decide to stop user from rooting or side loading then we can't do anything about it.