It turns out that this is due to an error in the Fraction context that doesn't properly insert parenthesis in some circumstances when the fraction is turned into a string. Since you use
$ans = Compute("$F+C");
The value of
$F is first turned into a string, inserted into the larger string, and then re-parsed to form the new MathObject. Because the parentheses are missing, it is equivalent to
$ans = Compute("(x^2/7)+C");
(x^(2/7))+C, leading to your issue.
One solution would be to use
$ans = $F + "C";
$F is already a Formula object, it will handle the
+ by doing addition of Formulas, first converting the string
"C" into a Formula, and returning the new Formula that is their sum. This avoids the stringification and re-parsing that your original method uses (so is also more efficient).
See if that does the trick for you.
I've made a pull request to fix the problem. So a second solution would be to download the updated
contextFractions.pl file from that PR and save it to your course's
templates/macros directory (or to the main
pg/macros directory if you have access to that).