Datastudio: Blended Data: Datetime can't be aggregated

I have two tables case and party. Both of them are from BigQuery. In the case table, there is a date field in the DATETIME type in BigQuery. I want to plot a chart to show the number of cases per month. It works perfectly when I only use the case table.

However, when I blend the case table with the party table, the aggregation doesn't apply on the date field anymore.

This is how I join the tables.

How do make the aggregation of date & time fields in blends work as normal?

Update

I have tried to create a custom field with TODATE(date, 'RFC_3339', '%Y-%m-%d'), but the aggregation is still not applied to the custom field.

minimal example

1. Create a.csv in local.

a_id,date
1,2022-05-25T00:00:00
2,2022-05-25T00:00:00
3,2022-06-01T00:00:00
4,2022-06-02T00:00:00

2. Create b.csv in local.

a_id,b_id
1,1
1,2
2,3
3,4
4,5
4,6

3. Upload a.csv and b.csv into a Data Studio report.
4. Blend a.csv and b.csv with inner joining on a_id field. I removed "Record Count" from metrics in both tables and added all fields as dimensions.
5. Add a "Table" type chart using the blend created in the previous step.
6. Put date field as the only one dimension and select "Year Month" as the type.
7. Put a_id field as the only one metrics and select "Count Distinct" as aggregation.

actual result

expected result

example report

Here