AFAIK the older technology couplers are susceptible to problems like these unless stabilized properly with formalin. Insufficient fixing would result in the typical tan to yellow staining and bronzing you'd also get in B&W materials. Colorful staining means the couplers themselves are involved.
I've never seen an issue with XP1/2 or Super except when someone has over-used their Blix leaving a trace of silver halide in the emulsion. While the C41 process should use a separate Bleach, then Fixer some kits use a combined Blix.
When Ilford sold their own kits for XP1 there was no stabiliser, and no C41 kits use one, but you washed the films after Blix, or Bleach then Fix.. When Kodak and later Fuji introduced washless C41 machine processors they added a stabiliser instead but not Formaldehyde based.
It's worth noting that small labs have been using Jobo processors for at least 20 years, I visited many labs for work. 20 years ago some prosperous minilab offering C41/RA4 would use a Jobo system for E6 and/or B&W reversal. More recently photo shops are using smaller processors, Jobo and similar for C41, ECN, E6.
Ian