Sorry to hear that you are having some issues. Maybe I can help out a bit! The issue seems to be more DAW related since you are also having some issues with Automation and plugin parameters. Typically when adding plugins, stoping playback first is a good idea like what was previously posted. If your session is a bit larger, increasing your buffer size may help during playback as well even if you have a good amount a ram like the 16gb you currently do. A good way to test if the Scarlett 18i20
drivers are a factor, is to set the Audio Output Device of your DAW to "Built In-Output" and add a plugin during playback. If the issue persists, then you know it is the DAW. However, if it does not please contact us directly here : https://us.focusrite.com/answerbase/contact
and we will be happy to help you! Hope this helps shed some light.