The Apache shade plugin allows maven builds to create an uber jar that contains dependencies in the project. In addition, the shade plugin allows you to relocate dependencies into a private namespace to prevent class conflicts on shared class paths. Avro does not support relocation.
All generated Avro objects contain a string field named SCHEMA$ which serves as the authority for the class namespace. When the shade plugin updates the byte code to relocate the class, it doesn't alter the SCHEMA$ string. This break Avro use of reflection since the namespace in SCHEMA$ points to an incorrect location.
I spoke with Doug about the issue and he was kind enough to provide a quick hack in order to fix this issue. The hack is to check for mismatches between the byte code and the SCHEMA$ and, when they don't match, to defer to the byte code. I'll attach Doug's patch to this Jira.