Edit: although it doesn't make sense to me for play store to do the same without the source code available
Edit 2:
The reason is that they forced new apps AND apps for Android TV to use App Bundles https://developer.android.com/guide/app-bundle
This type of release cannot be installed as it but can be used to generate the apk files. In order to do so, the Play Store has to sign on the fly.
Not buying it. They could let the dev sign evey combination before uploading. They'll be caching them anyways