I have never dunked a Manor Bag, so my recommendation was based on an assumption that there was no cardboard in the flap. I can't say for sure that there's no cardboard in an authentic one, but I would be surprised if there were.
@nursebetty's post quoted above said she had dunked two of them, and only one of them had a problem. And here is a post by
@Transported who successfully dunked one:
Coach Rehab and Rescue Club
The only
Coach bag I am pretty sure has cardboard in it is the Binocular Bag. I dunked one of those and it came out fine, no wrinkling. As with all rehabs...YMMV!
.