Round deck plate or rectangular? Either way, I can easily understand why a crack would originate there. Any feature that has a sharp edge or juncture, like a corner, cut, sharp radius, or other in an otherwise "uniform" surface, will be a source of a stress riser. That's a rather fancy way to say that where's the maximum loading will be absorbed and where some weakening or defect may occur.
Then too, the tensile (read strength) of gelcoat, although very thin, is like 10 times that of the layed up glass substrate. In other words - it is very brittle. What causes cracks is that the substrate flexes, but the harder gelcoat cannot flex as much, so it cracks. This is what causes stress cracks, radius cracks, and cracks from impact damage. It is not necessarily that YOUR floor is unsupported correctly or that there is an inherent defect. Note that voids under the deck in the layup can cause cracks too, but I highly doubt that would ever occur on the Parker-built hull.
We are seeing far more gelcoat issues on boats today (ALL brands) than we ever did in the past primarily due to 2 issues IMHO - (1) the gelcoat itself and (2) financial pressures at companies, causing "time" issues.
Issue #1 occurred due to ever evolving EPA changes for chemicals that out-gas as a byproduct of the production of or during the use of the gelcoat or hardener. Much of this has been EPA mandated emissions of volatile organic compounds. My friend who has a f'glass company and specializes in boat repairs (where I get this info) has showed me colored hulls by premium boat companies, which have chalked in as little as 3 years. In my friend's opinion ... the gelcoat we have today is junk compared to what it was years ago. I bet Parker uses the best they can ... this is an issue they cannot control and it even plagues PPG, the largest player in the gelcoat world.
Issue #2 can happen when boats are popped out of the mold too soon. If you look at a few year old Angler and see long longitudinal stress cracks around all of the deck hardware ... this is because the hardware was torqued down the day the hull came out of the mold. Boats used to sit in the mold to cure for at least a week. Boat companies can't afford to do that today. Though the surface glass might be cured, inside the layup, the mix is still "green" or not fully cured, which can take up to a week. Companies cannot afford a boat to sit idle that long - just is. But I would bet that Parker leaves their boats in the molds longer than anyone else in the business and doesn't torque deck hardware down until fully cured. That's just my guess ... as their boats are just so darn tough!
So ... how to fix?
Bried example, you're going to have to take a dremel tool and grind out a shallow V in the crack, and then fill with gelcoat, thickended if/as needed with cabosil.
See what Parker says, if you don't need much, I can send you some of both products though you will need to obtain some hardener. Now I don't have the time right now ... but in the search feature there are a few posts detailing the process for gelcoat repairs. Search the projects forum.
Good luck and please let us know how you make out!