Yeah i agree im sure it is to do with the noise in the YB bay, but that's why my YB looms are shielded in the way they are, because they are still in the same bay they were 25 years ago.
They do seem to become even more picky about noise when converted to wasted spark too, i think many of the issues people see with the WS kit are down to poor Looms, and unsheilded Looms, it may even be down to the extra RF from the HT leads etc.

For what it costs extra to shield the Map i wouldn't even consider not doing it on them TBH,