I’ve only ever seen this caused by the theme changing, globally, the CSS box-sizing to border-box rather than changing it only when they need to. I, personally, do not think this is good practice since it will break anything expecting the browser default. Perhaps you had them update at the same time, relatively. It is possible another plugin to do this, but I haven’t run across it yet, just themes.
Which version of Tile Plus do you have installed? It has been updated a few times since your initial purchase and one of the updates included a fix to force the default for the template.