BigQueryBatchFileLoads dynamic destination

I am trying to use BigQueryBatchFileLoads to upload data from pubsub. It works fine for a single table, but when I try to use a dynamic destination such as destination=lambda elem: "my_project:my_dataset." **** elem["sensor_key"], it just makes a new table for each time the triggering_frequency procs. I know it makes temporary tables before loading it all into one, but it never loads them. It just creates more and more tables.

Imported from Jira BEAM-9917. Original Jira may contain additional context. Reported by: tordaut.