The keep rules in https://github.com/square/retrofit/blob/master/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro are insufficient for R8 in full mode.

See for example https://issuetracker.google.com/issues/120168590#comment3 and https://issuetracker.google.com/issues/122940365#comment9.

As a simple example, consider the 'SimpleService' sample:
https://github.com/square/retrofit/blob/master/samples/src/main/java/com/example/retrofit/SimpleService.java

The interface 'GitHub' is never instantiated in the code (except via reflection), and it is not explicitly kept by a keep rule, so R8 can safely replace all values in the program that have type 'GitHub' with constant null.