I have never tried limiting the screen notifications with LightFlow.
The way I have my notifications setup to work with LightFlow Lite is like this:
For the particular app, screen notifications are on to popup but sound is turned off.
In LightFlow, the sound is turned on and the screen popups are turned off.
This gets me all the popups in the notification bar while limiting the sound to once every 10 seconds.
I am running LightFlow Lite 3.99, Android 8.0, LG V30. These days, the few remaining apps I have controlled by LightFlow are the ones that do not let me pick the notification sound from within the app.