One MRI positive, one MRI negative?

Moritz, welcome!

Yup, definitely a confusing situation. Have you had a diagnostic mammogram and ultrasound more recently than 5 months ago? If not, I would think that might be an option of what to do next, to see if any correlation can be found to the finding from the first MRI. If something is there, it might not have been visible on the ultrasound and mammo 6 months ago, but it could show up now. And now the radiologist knows exactly where to look. But if you have had a more recent mammogram and ultrasound, I'm not sure what you can do. A Radiologist can't biopsy something if they can't find it on imaging while doing the biopsy - the needle placement has to be very precise and the imaging done during the procedure is used to guide the needle to the right location. The radiologist can't just go off the previous imaging because even being 1mm wrong on the location would result in an unreliable biopsy result.

The good news is that ADH is a high risk condition and DCIS is a non-invasive condition, so what was suspected from the first MRI is not urgent. That would suggest that waiting 6 months and having re-imaging then might be another answer. Getting advice from a breast surgeon is probably a good idea - PCPs are often out of their depth on this sort of stuff if there is anything out of the ordinary going on.