*istD overexposure with M50/1.4, but not M50/1.7

[Peter Smekal]

This time I only tested two lenses. *istD M mode + the green button.
Aperture 8 in both cases. Same scene. M50/1.7 works just fine, M50/1.4
gives totally washed out overexposures. Any clue?
Peter