No worries Mick - maybe more information will guide us to proper solution :)

1. No, it didn't. We used R8 for some time. I tried to track any recent changes in our configuration, but nothing obvious popped up. I also tested previous versions of R8, but each library above 3.0 produces same issue, while versions below 3.0 throw java.lang.IllegalAccessError. There is probably some correlation between our R8, Gradle and Support SDK versions.

2. Mostly it's our custom code. The case is like this:
2.1 We perform http request with our custom library, that is a wrapper around OKHTTP
2.2 We pass in that request expected response type
2.3 Once we receive response, we use getClass() method on responseType object passed from called request in order to properly parse newly received responseData object.
2.4 During this step, we should parse given object, but we receive entirely different Type from getClass() method, so process breaks here. To visualize error level, we expect package com.company.core.api.framework.implementation, but we get instead com.company.core.api.standard.model.