When the tests for EIC were introduced we tested all our corgis at the time 9 were clear and two carriers. Since then we have selectively tested pups and recorded the results.
In our testing it is apparent that the gene could well be attached to another gene as in one litter the red colour dogs are clear and the tri colours carriers. This could be a coincidence but it has flowed through with ongoing testing.
I have a found a website from the University of Minnesota which tells it better than I could – here is an extract followed by a link The research laboratory has tested 100-400 samples from each of the common retriever breeds – they have tested Golden Retrievers, Flat-Coated Retrievers, Chesapeake Bay Retrievers, Nova Scotia Duck Tolling Retrievers and Curly Coated Retrievers. They have also tested numerous dogs from many non-retriever breeds. So far, the mutation has only been found in Labrador Retrievers, Curly Coated Retrievers, Chesapeake Bay Retrievers, Boykin Spaniels, German Wire-haired Pointers, Old English Sheepdog, Cocker Spaniels and Pembroke Welsh Corgis. https://vetmed.umn.edu/research/labs/canine-genetics-lab/genetic-testing/exercise-induced-collapse