Validation Rule on Read-Only Fields

Validation Rule on Read-Only Fields

Hello everyone,

We want to use the "Street" field for a duplicata check and in order to make sure the address is written exactly the same from one rep to another we require them to input the address using a Button that makes an API call to Google Maps and then fill the fields.

I configured the "Street" field as Read-only for all profiles except admin and configured a validation rule so that when it's empty there's an error message.  Last week, the validation rule worked : if the representative didn't use the button and left the "Street" field empty, he couldn't save the record. But this week, he can!

I'm guessing that's because the field is Read-only for his profile, so the validation rule is not triggered. Is there a way to make it work? 

Thank you!