Strange crashes reported by Crashlytics on a Google "Mainline" device?

Question

On almost every Play Store release of my app, I get weird crash reports of certain resource files being missing. Decompiling the APK shows the resources are there. The crashes are always by 1 user on 1 device.

The puzzling thing is the device model name is "Mainline" and the manufacturer is "Google". Looking around on Google, there is no device with the model name "Mainline" however there is a project named "Mainline". Does anyone know what is causing these crashes? Is Google running some weird test?

Screenshot from Crashlytics:

Answer 1

Google Mainline isn't an actual device. Mainline is a Google project.
MTS is a Google test environment based on the Mainline. The error could happen in the test process.
In my case, the ERROR_MISSING_INSTANCEID_SERVICE error was related to the lack of Google Play service. This makes sense to me.