This code is not enough to understand the problem.
This works absolutely right. So maybe you have a race condition or a validation on some object attribute or the a setter that is manipulating the value?
By comparison, when the printer doesn’t work first check that the cable is plugged in and getting power, before attempting more complicated things such as diagnosing circuit boards.
The string values should be converted to numbers. That way addition will add numbers, instead of concatenating strings.
I count myself to be in your same company for I didn’t even think of it at first, having to log out ((lb / 14) + st) before even thinking of that solution.