Python's Test Expansion Service for Combine transform returns pickled coder

Trying to use the combine transform of Python's test expansion service executed from the Go SDK fails during runtime. Crashes the Go SDK harness with the following error:

unmarshalling coder VSkoscjkPN_Coder_TupleCoder_7_length_prefixed_windowednmarshalling coder VSkoscjkPN_Coder_TupleCoder_7_length_prefixed_windowed unmarshalling coder VSkoscjkPN_Coder_TupleCoder_7_length_prefixed unmarshalling coder VSkoscjkPN_Coder_FastPrimitivesCoder_1_length_prefixedcould not unmarshal length prefix coder from spec: component_coder_ids:"VSkoscjkPN_Coder_FastPrimitivesCoder_1" , want a custom coder as a sub component but got spec: 

Imported from Jira [BEAM-10796](https://issues.apache.org/jira/browse/BEAM-10796). Original Jira may contain additional context.
Reported by: kevinsijo.