Alert rule: "Query Returned No Series", however testing shows non-zero row count

I’m not sure if I am hitting a bug in Grafana or there is a misunderstanding on my part.

I am attempting to build a panel with an alert setup that shows a week over week ratio of event counts. I have a SQL query that uses two sub-queries to build up “this week” and “last week”, and the query joins these to produce a ratio.

My query is functional in the graph view and returns data as expected. However with the alert setup, testing the alert rule shows:
Condition: Eval: false, Query Returned No Series (reduced to null/no value)
The Testing Rule window also shows a non-zero row count returned from the SQL query, but the “series” is a zero length array.

I can run the SQL query shown in the Testing Rule window in an external DB tool and it return the expected window of data.

I am not using any template variables in my query.

Any insight or help is greatly appreciated.