In my case I'm implementing an On Demand Module (https://developer.android.com/guide/playcore/feature-delivery/on-demand) and all resources missing were inside a 3rd party SDK dependency I needed to add inside the On Demand Module...strings, styles and xml files were missing. To solve this I simply added the missing things empty inside the main module as suggested here:

https://alecstrong.com/mytalks/edfm/

Inside the video check the minute 25: Gotcha 2 Manifest Merging.

Here's the web page mentioned inside those slides:

https://medium.com/androiddevelopers/a-patchwork-plaid-monolith-to-modularized-app-60235d9f212e

Check the subtitle: "Styling issues"

Thank u Alec Kazakova and Ben Weiss for sharing the struggle...I wish Google did more with this kind of issues... troubleshoot their messy solutions is a nightmare