A looked at the settings in the fixup. You are using the Convert Colors fixup and constraining the color conversion only to objects that are in the registration color space
That color space is a special color space which will cause equal coverage on all channels (if you were converting to CMYK+O, for example, the output would have equal coverage on all 5 channels. You cannot use color conversions to change registration to K only CMYK.
Acrobat DC has a fixup for controlling conversion of the Registration Color Space
As with other fixups, you can control the registration color space objects that are converted by checking them against a preflight profile.
You can also control handling of overprint
I've enclosed a preflight profile that will convert to K Only CMYK and leave Overprint unchanged.
Received the following question via email
Could you load my workflow and tell me what I missed? I wanted it to change registration color to black on the attached file. Didn’t do anything to the black.
I included small version of the file and the exported Acrobat preflight fix.
On a positive note, it looks like the customer is going ahead with FF Core.