You should also countersink the mounting holes for any sealant that will be under extreme compression to work well. The advantage from butyl is that is never dries and stay elastic to take up some movement. However, if it is compressed out, it won't work either.
Ya, I did, on both the wood mounting block and the deck itself.
I'm only making assumptions on why it leaked, I was shocked when I pulled the fitting out, both the mounting plate and clutch were difficult to remove due to the butyl and the bolts were covered in it. I can't figure out how water would have found it's way in.