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First off, like Stefan, I'd guess the mask is mis-shapen, mis-centred or at a slight angle.
What does your Airy disk image look like either side of focus? How round are your stars near the edge of the field and is focus/curvature/aberration symmetrical left-right and up-down?
TBH, though I do have a Bahtinov mask for my ED80, I don't use it except sometimes at the start of the night for initial rough setup. I focus by eye these days and before that (when I used a DSLR) I used BYEOS' FWHM measurement.
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