It seems that math regions don't work inside text regions, at least not well enough to show the answer. Smath seems to be doing the math in the background, but it displays an error in the region saying variables are not defined. When entering the variables into the math region, the pop-up hint shows the value, so it is clearly defined. Also, if you print the value of the variable it shows it too.
This example has three different text regions that contain math regions within.
Math Regions in Text Regions Glitch.sm (4 KiB) downloaded 54 time(s).