When a tag is defined with a default value (e.g. on a Landing Zone), the default value is not actually applied to existing entities right away. The Landing Zone table view already shows the default value as if it were set, but the value is only really applied once you open the entity and press "Save", even without making any changes.

This becomes confusing when the tag is used as a Building Block input: the Building Block reports the tag as missing and appears to be "waiting for input", even though the table view already shows a value for it. There is no indication that opening and saving the entity is what's needed to resolve this.