Are partitioned jobs tags are not propagated to jobs created with backfills? If I run a job manually (for a single partition) it has the proper custom tag. If I backfill multiple partitions the jobs created for them don't have the custom tag. The tag I'm missing is defined in my location python code.

the tag is defined on the job definition itself? That sounds like a bug - mind filing an issue?

Yep. Will do.