I can't see any reason why Avast would want to reset existing font settings.
I could see something if it was actually blocking the System Font Size Changer and Advanced System Font Changer programs, in which case I would expect you to be given an alert or notification on those programs.
I haven't used either of these, so I don't know how they work to make the changes. e.g. is it just changing the settings once or is it doing this on the fly as screens are opened ?
I have a Win10 Home 1080p 15.6" laptop screen and the OS default Display Scaling setting is 125% and Resolution 1080p, but I don't like that and set it to Scale to 100%. That makes the text pretty small for browsing so I set my browser Zoom to 133.
Now this might sound a bit convoluted, but Avast doesn't mess with these settings.