I have Google Analytics UA linked to BigQuery and I see there are lots of generated data having NULL value in a specific custom dimension.
I was trying to understand under what circumstance this could happen? is it a bug in BigQuery or is it something our developer should check as to why the custom dimension values were not set?
As mentioned by @Dmitry in the stack link, it can be normal to see NULL
events in your BigQuery. You can reproduce this by constructing a hit using measurement protocol omiting the category, action and label parameters but specifying custom dimensions and metrics.
The hits from older versions of GA might record in BQ as nullable events.
Still if you need any clarity on the issue or the detailed investigation, I would suggest you to create a new GCP support case. Otherwise, you can open a new issue on the issue tracker describing your issue.
Posting the answer as community wiki for the benefit of the community that might encounter this use case in the future. Feel free to edit this answer for additional information.