One of the underwriter's questions was "Do you have any surgeries scheduled or planned for the coming year?" I said, "Yes, I'm planning to have cataract surgery on my right eye." (I'd already had the left one done.) They flat turned me down just for that. I was shocked that they would do that for something as routine as cataract surgery, on only one eye, but the letter from the insurance co. made it clear that's what it was!
I think it's bonkers that the aca rule about not disqualifying people for pre-existing conditions doesn't apply to Medicare supplements, but the reason I was turned down seemed doubly bonkers to me. I went ahead and had the surgery with only my Medicare parts A & B coverage, and the amount I had to pay out of pocket fortunately was an amount I could handle. Applied for a supplement again the following year and was approved with no problem. Nothing else had changed except that!
I can't remember the details, but I know the reason we had to go through underwriting was because we waited too long to apply after my husband "retired" (quotes because he was actually laid off, but it ended up turning into his retirement). I was told that if we had applied for a supplement within a certain amount of time after the coverage from his employer ended, it would have been an automatic thing.
I know none of that info helps you, and I hate that you've gotten caught in this bind. But maybe it will be of use to someone else!