It is a common occurrance that has been reported for several Samsung android devices. Theory is power saving is somehow turning off the avast permission so you have to regularly turn it off then on again. I see it after updates and sometimes in mornings.
See this old thread: Scan says Webshield off but Accessibility toggle for Avast is on
You may or may not see a notification that webshield is disabled, so to be safe check resolve Core Issues on Avast Mobile App.