ClassCastException: org.apache.avro.io.parsing.Symbol$Alternative cannot be cast to org.apache.avro.io.parsing.Symbol$UnionAdjustAction

I've got a complicated schema that when deserializing data gives me the following exception:

java.lang.ClassCastException: org.apache.avro.io.parsing.Symbol$Alternative cannot be cast to org.apache.avro.io.parsing.Symbol$UnionAdjustAction
at org.apache.avro.io.ResolvingDecoder.readIndex(ResolvingDecoder.java:188)
at org.apache.avro.io.ParsingDecoder.skipTopSymbol(ParsingDecoder.java:66)
at org.apache.avro.io.parsing.SkipParser.skipTo(SkipParser.java:71)
at org.apache.avro.io.parsing.SkipParser.skipSymbol(SkipParser.java:93)
at org.apache.avro.io.ResolvingDecoder.doAction(ResolvingDecoder.java:207)
at org.apache.avro.io.parsing.Parser.processImplicitActions(Parser.java:116)
at org.apache.avro.io.ResolvingDecoder.drain(ResolvingDecoder.java:141)
at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:72)
at org.apache.avro.file.DataFileStream.next(DataFileStream.java:195)
at org.apache.avro.file.DataFileStream.next(DataFileStream.java:182)

If I go into ResolvingDecoder.readIndex and modify the code to use an instanceof check, my data deserializes perfectly fine.

I can send all files and unit tests needed to duplicate the error to whomever is working this ticket