[NTG-context] lmtx and \prime ?

I get a different result for mkiv and lmtx for the following input: \starttext Even now, a six footer is two standard deviations from the mean. Remember, 5\prime{}7\prime\prime{} is 50th percentile today for a 15 year old. \stoptext With mkiv, the prime marks for inches and feet are where I expect them; in lmtx, they are placed almost to the baseline. Wrong lmtx, or user error? Mike