I wish to apologise for my previous post, and amend it. I was in a bad mood, but there was no need for me to take it out on forum members. It should have read more like this.
As far as I can see from the photos the transom is split on the interface between the moulded top return of the deck liner (Ribraff care to take a bet, pint says itís moulded.) and what ever the builder has used as a filler/adhesive.
As the transom has bowed it has created tensile loading on this joint which has failed to some degree resulting in the visible damage.
This is possibly due to the over use of some form of bonding paste or air voids in behind. At the end of the day, no one can say for sure from a photo, and the only way I know of examining a build thoroughly is destructive.
Give the builder first chance to sort it but if it reoccurs then get it to a qualified third party for a report.
My two concerns with this would be why is the thing is flexing to that extent in the first place, and water ingress into the transom plywood.
If I can be of any help, call 01548 856078.
But I may be talking Rubbish.
Expurt is a drip under pressure, and the difference between an Amateur and a proffesional is getting paid.