[Firebase] DAU vs Unique user_engagement User

[Fia Shao]

Dear，
As I know, the DAU from the dashboard should be the unique user of user_engagement event. 

But we encountered a problem that one day, the number of user_engagement was stable, but the DAU from the dashboard suddenly increased a lot. After analysis, it was found that some of the events we uploaded were counted in DAU,and the country was "not set". The events were uploaded for Google UAC ads. 

what could be the reason？