I have hooked a few trout on top that I would say took the fly as you described, but I was in the vicinity of fast water and never heard it.
I did however see the fly simply vanish into a hole in the water, one I landed(18") and the other straightened the hook.
Both times i am happy to say resulted in a solid hookup.
I would love to see a video though😉 CJ
It's possible that since the fly is tethered to a leader it doesn't have the "freeness" to actually make it all the way into the trouts mouth.
You sound convinced that these fish have figured out that they can be a significant distance below the surface and coupled with their personal gill ability can pull a bug under and into their mouth without being at the surface.
If they have it figured out this close, and accurately, it would work with any bug not attached to a leader, but a leader would inhibit the flys movement maybe enough to not make it all the way down the vortex.
Just speculating here...I do all my fishing with nymphs, and don't deal with the same frustrations you guys do. I just don't see all the subsurface mistakes I make, and I'm happy like that😎 CJ