In ArcGIS Online, the system-generated Creator field assigns a blank value for anonymous users, rather than <Null>, which makes the field unusable in other applications.

Additional Information

Workaround

In ArcGIS Pro, change all the blank entries under the system-generated Creator field for NULLs. To do this, follow the steps: 

1. In ArcGIS Pro, open the hosted feature layer.
2. Create a new field in the layer. Name it as Creator2.
3. On the layer, run Select By Attribute where Creator is equal, and then leave the entry blank. This selects the entries by the anonymous users. 
4. Then, leaving the selection, right-click the Creator2 field, and select Calculate Field. Enter Creator2 = Null. This assigns <Null> in the Creator2 field for the anonymous entries. 

After applying the steps, the 'is not blank' filter in ArcGIS Experience Builder works. 

Steps to Reproduce