[NTG-context] Strang behaviour of apostrophe in math environments

Hello together, the code is simple: \setupbodyfont[iwona,10pt] % also tried with antykwa \starttext $ y^' = y' $ % notice that I also used a Circumflex on the left side of the eq; same below \startformula y^' = y' \stopformula \stoptext When I compile the code above, the apostrophe on the left side of the equation isn’t showing up when iwona or antykwa bodyfonts are used; it only works when using regular or sans bodyfonts. Is this behavior expected? I am wondering because when you zoom in with regular or sans bodyfont, there is a difference in vertical spacing between Apostrophe with and without Circumflex. Thanks in advance, Sebastian