I have an article somewhere about mottle on the base side. I will look it up. It referred specifically to BTZS tubes but would be analogous to what occasionally happens to sheet film in any similar process. In some cases it is simply that there are some streak-shaped patches where some of the dyes have not come out - again due to direct contact with the tube wall. In other cases this dye residue is observed along with a slight difference in sheen. But I believe these defects can be remedied. The author of the article (Phil Davis) also indicated these defects almost never print.
Regarding DDX, it is very similar to TMax RS in its working properties. If you are processing properly with your Jobo there should be no need to switch to DDX or any other developer for that matter. Notably John Sexton uses a lot of TMax RS to develop TMax sheet films in his Jobo.