For KMB, I think the problem that you are describing is the fact, that there is space between the reel seat and the rod blank. Occasionally, a reel seat is too large for the blank in the area that one wants it to be seated. With that in mind, the blank area must be built up and yes, there is spacing between the ends of the reel seat and the blank. This area must be filled and tappered.
St Croix tends to use CARDBOARD to fill this space. I just had 2 rods repaired due to the reel seat failing and twisting when under torque of a fish.
Most custom rod builders will use a material suitable for the task. Cardboard is not suitable.
The only way to avoid that problem is to have every reel seat fit perfectly to the blank in the location that it is to be mounted. That my friend is not going to happen 100% of the time. But, in my 45 yrs of fishing, I have found the custom rods to far exceed production rods in all but a very few times.
I own several St Croix rods. The reel seat has failed on all of them that were production rods. The rods that were built on St Croix blanks by custom builders have had no reel seat failures.
You gets what you pays for.
American by birth....
Southerner by Choice.