When using the Query Widget, there is an option that allows to "Create a new layer every time the query is executed"
When created, the new layer appears at the top of the Layer List widget as a new layer that can be turned on/off.
Then, when running a new query, a new layer is being created and added at the top of the layer list.
The previous query result layer is still available in the layer list BUT it has been turned off automatically when the new query was run!
Ideally, the previous query layer would remain turned on by default - or there would be an option to allow this. Otherwise, if the user is running multiple queries and wants to be able to view all the results, he will need to turn all the layers back on.