I think I've figured this out...
I've included the MUNICIPALITY field in my Validate_Attribute_Lookup method.
It seems that the Attribute Assistant will add a trailing space to a Street Name when the MUNICIPALITY field value is blank.
So, in cases where the MUNICIPALITY was not calculated prior to the method running, I see the white space.
To resolve this, I set a higher-priority, Intersecting_Feature method to confirm the MUNICIPALITY directly prior to running the Validate_Attribute_Lookup.
It has been working fine, ever since.