I do not understand this issue. The intent of the new regulations from this year is clearly for worldwide deployable Soldiers with no functional limitations that interfere with basic warrior tasks. Given the sweeping changes to readiness with the July 27th update to regulatory guidance, I just don't see how the situation you are describing could be justified.
AR 635-40: I would question what regulatory guidance (specifically) is being used to make that fit determination. Note that the only verbiage I am aware of regarding special processing requirement for MC officers involves mandatory review (AR 635-40, Ch 4-25) in some circumstances. There is no longer any specific verbiage regarding MC officers being found fit/unfit that I am aware of. Note the specific verbiage in Ch 5-4 (c) 1: "As a minimum standard, the functional tasks listed on DA Form 3349 will be considered common military tasks required of all Soldiers." The words "license" and "credential" do not appear in the DES regulation. There is discussion about duties commiserate with rank, etc, however this is listed separate from the clearly delineated minimum standard for fitness, which includes those aforementioned functional areas.
AR 40-501 (updated 27 July 2019): This sounds like it was not the issue; you mentioned you were found 'unfit' at MEB, I believe a more accurate statement was that you were found to not meet the medical standard for retention, which by definition (since DES is the one saying it) means that you do not. So it is now a fact that you do not meet the medical retention standard per AR 40-501. If the reason you do not meet the medical retention standard was due to inability to perform basic warrior tasks IAW STP 21-1 or any of the functional tasks listed in DA Form 3349 (the 'checkboxes'), then AR 635-40 clearly states that you should be found unfit.
DA PAM 40-502: There is additional regulatory guidance in this new instruction (Jul 27, 2019) that probably isn't the issue either, given you made it through the MEB phase appropriately.
I would question, in this climate of 100% readiness for all Soldiers, including physicians, how the PEB is meeting the intent of the applicable regulations by finding you fit given you do not meet the medical retention standard. Additionally, given the clear mandate to force shape the medical corps, such as action merits further scrutiny in my opinion. I would be curious to know what regulatory guidance is being used in reference to credentialing/privileging/licensing as sole factors in finding an officer fit.