If you are at the limit of the buffer's capacity (which can take place at the densest part of the negative) then you can get a downdurn. This may be a case of film and developer being built together to give the proper curve and image quality and therefore your developer here is overdoing some aspect that should be more restrained.

If that is the case, then I would expect some degradation in the image structure in your developer wrt the XTOL.

That is what I am guessing at this stage. Your developer is overdoing things and should have less buffer. In its present state, your developer will suffer in grain or sharpness or both wrt XTOL.