I have never identified a specific allele for that problem but it is not uncommon in selfs. It's definitely genetic, and in my experience, white spots on the footpads were frequently paired with white toenails or even a partially or wholly white foot. I had no reason to suspect either vienna or broken (it was never enough to make a rabbit look booted), but it may very well be connected to that chin allele.
Way back, I had an otherwise fabulous black Satin doe, Onyx, that had a white toenail. She was so stunning that used her for breeding, and for generations I had all three marking issues I listed above cropping up in her offspring. In retrospect, Onyx was probably a self chin herself, or at least carried chinchilla (her granddam was a broken chin). Many of Onyx's descendants were self chins, both blues and blacks, but I did not realize it until about four generations down, when I bred one of her blue great-great-great-grandsons to a NZW...and got an entire litter of chinchillas! (The NZW carried or was agouti, which allowed the chinchilla in my "self blue" buck to be expressed. I've since learned that NZWs very frequently carry agouti <
A>, chinchilla <
c(chd)> and/or steel <
E(S)>.)
The good news is that after choosing my broodstock very carefully and never accepting the slightest hint of a footpad spot in my breeders, I very rarely see white footpads and never have bunnies with white toenails anymore. So I assume the spots came from some modifiers or other genetic tweaks linked with that chinchilla allele, which I eliminated by selection without eliminating the chin. As far as I can tell from my records, I never had footpad spots appear in kits outside the self chin line (though I can certainly believe it could happen).
Maybe
@reh has something to say about this?