If you are getting small reproducible roundoff error values like 0.40625 and 0.4375 then it is likely due to testing a value near the limits of what that FFT size can test. In other words, nothing to worry about.
If you are getting roundoff errors like 0.49999 then it is a hardware issue.
P.S. The FFT limits changed ever so slightly between 26.6 and 27.7. So it is entirely possible that the same number would use a different FFT size in the two versions.