Reshuffle seems to be dropping elements in streaming Dataflow pipeline.

https://stackoverflow.com/questions/62017572/beam-java-dataflow-bigquery-streaming-insert-groupbykey-reducing-elements

This showed up on SO and it shows a GBK in a reshuffle actually dropping elements. I asked the user if they've tried it on other runners, and from what I understood they tried with the Direct runner but never got to the point in the pipeline where the original issue occurred, so it's hard to tell if this is a runner or SDK issue. The user has also contacted Dataflow support.

Not sure if this is high priority or not or who would be an appropriate assignee, so I'll just leave the bug unassigned for future reference, so if this pops up again there's evidence that it's been a problem.

Imported from Jira [BEAM-10228](https://issues.apache.org/jira/browse/BEAM-10228). Original Jira may contain additional context.
Reported by: danoliveira.