You will always see some color in the Dmin of RA4 paper developed in RA4 developer. It is inevitable. There is always some tiny amount of fog in any photomaterial that shows up with development.
To prove this, you can take some MGIV paper and fix and wash, and another piece and Develop, stop, fix and wash. You will see a difference there as well. It may only show up with a Densitometer, but it is there.