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First, you have a discrepancy between the Mom's Rh type on the pre- vs. post-delivery specimen. That needs to be resolved just as you would need to resolve an ABO discrepancy. I would suggest that a new specimen be collected from the Mom and tested. If the new specimen's Rh type agrees with the pre- specimen, then it would indicate there was a problem with your post specimen either misidentification or contamination. Repeat the rosette test on the newly collected post specimen. If the new specimen's Rh type agrees with the original post- specimen then you have your answer that the rosette test is false positive due to the Mom having a weak expression of D which interferes with rosette testing. You are not detecting Rh + fetal cells, instead you are detecting Rh + (weak) maternal cells which would explain why the rosette test is positive but the KB stain is negative. You would also then need to follow up as to the pre- sample and whether it was misidentified at collection, etc.

Another reason why our computers are better at selecting ABO compatible units than serological testing is.

Another caveat about doing titrations on the Vision is that it always runs all 10 (or 12?) dilutions. That will burn through a lot of reagent cells unnecessarily on a titer of 4! I agree with those above that it is critical that the OB/GYNs know that you are using a method that gives different results than their textbooks are based on. Every gel titer result should go out with a comment explaining how its results correlate to the literature for further evaluation of the pregnant person. At least nowadays they are likely to follow with Doppler ultrasounds rather than riskier, invasive amniocentesis. I think a review of the CAP survey results is very enlightening.