That can happen when your theme sets no rules for it’s fonts. Some poorly made themes don’t do that.
Anything AdRotate does for fonts is applied to the advert parts only.
I don’t see why updating from 5.0.1 to 5.7 would cause this as this method has been in place from version 4 – Several years ago 😉 So that update shouldn’t make a difference.
But aside from that, in my experience, every time this happened the issue was with the theme not setting a character width or line height properly or something like that.
Umh, the Schema theme is very simple, so can you let us know something about it? If not, you should proceed with the refund.
It is not possible that the update of the plugin will change the text of a whole website. So the problem is most likely to be attributed to the plugin update. The theme clearly allows us to get our hands on fonts etc., but the plugin shouldn’t go to touch these settings.