this one helps. After looking into this, I found out that this is a problem if you first build without proguard enabled and then build it with it enabled. This is due to the incremental mode of dex. You can do a clean build after enabling proguard and it'll fix this.
fixed the issue. Will look into that further In my case the proguard parameter "-adaptclassstrings" caused this issue. After removing this from the config, the jacoco results contain the original names again, so the merging works as well.
Finding the obfuscated stack trace generated by proguard