Using the 'is not' operator in building expressions for the Filter and Query widgets result in incorrect queries that use the '=' function instead of the '<>'. This returns incorrect results from the intended expression.
The issue does not occur currently in the latest version of Portal for ArcGIS 10.6. The same expressions can be used and the correct results are returned. Images of the difference in ArcGIS Online and Portal for ArcGIS of the query results are attached.
The opposite of the workflow of using multiple 'is' operators inside of a set can be used in ArcGIS Online to return only the results that are desired.