Steps to reproduce,
1. Add sql server plugins (1.4.1) and driver(v6.0.jre8) from hub
2. Add sql server source to canvas and configure to talk to a sql server db
3. Once configured click "Get Schema" to get the schema for the specific table
4. Now change any property to an incorrect value (like source or import query) and click "Get Schema" button again.
Since the values are incorrect the "Get Schema" should have returned validation error but it seems like if there is an output schema in the validation request we don't connect to the db to get new schema.