We are seeing this same behavior, when built locally the apk can be opened and one can see the plug is listed in the cordova_plugins.js file however when the build ionic package build, when the apk is downloaded and checked for the entry in cordova_plugins.js it is not present.
As you have suggested I have rolled the spec back to 1.8.4, executed the ionic package build, downloaded the apk and confirmed that now the plugin is once again present.
I reported that bug to the ionic team and after the last recent update it is fixed.
In the meantime I updated to the last recent ionic cli (3.4.0) and the last recent cordova android platform (using this command: cordova platform update android@6.2.3 --save)
I cant figure out if perhaps one of that itself fixed the problemβ¦
But now phonegap-plugin-push 1.10.4 is included in my apk file.