I think that's probably a problem you'll find especially if dogs are diagnosed at an older age.
Yes this is a big issue. I'd say the majority of people do not contact their breeders when given a diagnosis, for a wide range of reasons -- they didn't stay in contact, they don't have an easy way to reach them, they are fearful of telling them, they have seen the reaction some have had from breeders (which ranges from disinterest to threats to rudeness). Fortunately there are some great, supportive breeders too.
The breed's real heroes are the breeders who consistently work to get as much information as they can about their own dogs and progeny and support their pet owners and breed carefully with more than just ribbons in mind.
The existing research shows a strong tendency for extended cavalier families to have affected or unaffected near relatives. The genome research also shows inheritance is a major factor, far outweighing other possible factors. Problem is it isn't yet understood.
in the US in particular, breeders are unlikely to know whether they are producing affected dogs unless they scan their dogs. Scanning still is costly there so neither breeders nor pet owners tend to scan unless they have a symptomatic dog or are really committed to research and getting info on their own lines. So while it may be that breeders believe they have produced few or no affected dogs, it is very hard for them to know what their dogs might be producing or whether they are themselves affected without scans having been done (and if more scans were done, more truly clear dogs would be identified, with greater hope for the breed as a result). I am quite sure my most affected dog, Leo, would have gone unnoticed til well into his breeding career had he remained (as was intended) part of the breeder's breeding programme. He had no symptoms for 2.5 years and many breeders would have begun using him at age 1 at stud (my breeder, now deceased, had asked to do so if I wasn't neutering him). He was a kennel dog and in with two dozen other dogs; I doubt his symptoms would have stood out until he was close to the age he is now (5.5). I imagine this scenario has been repeated many times in many places. And I know of cases where some breeders in the UK have sold on dogs they knew were affected to North America, where they continue to be used at stud.
These are amongst those arguing against scanning dogs.
I continue to be amazed that these people can live with themselves.