I tried this and it does reproduce for me too actually - it must be some Cordova bug as the documentation says the default content src is already "index.html", so adding it only re-applies the default. I guess the default got changed accidentally.
I've added the content tag for the next release to fix Cordova project exports. I've also patched the build server to add that tag when using cordova-android@10+, which means it should be fixed already in r261, since builds will now add the necessary tag on the server side.