Hi Ping,
Associations validations are working as expected like this. I refer to the demo microflows in the module:
Check if you need to add a validation for the association because it is empty:
if it is empty we add a validation:
You see here the Module name of where your entity is modeled in the domainmodel then the association name.In your case I think it should be Administration.Account_Accounttype 😇
It will correctly show the validation bar below the reference selector and no popup
Please let us know if this is working out for you 👍