1+a should be in parens: (1+a). This is a problem of display only. If you touch the result region and move the cursor around in it, then the parens are added magically.
The internal representation is 1/{3^{1/2}*(1+a)}
If you directly compute the inverse of the given expression, then the result is displayed correctly.
If there is no easy fix, I'd recommend to remove the Calculation menu. It does not create comprehensible documents, as the reader has no hint how the expressions are related. The danger of wrong results IMHO overcompensates possible convenience gains by menu access.