Not enough information to answer and not quite the right question (too simple). It's too general to say that fiberglass will never flex, or even to say that it should never flex. Could just be a weaker area of construction, which again is not enough information to even form an opinion on whether its a case of shoddy construction or not. Sounds like a typical area on a boat where as a designer one may not necessarily need or want the weight of a more bullett proof construction method.
That said, often the solution to add stiffness to an area with a minimum of added weight is to core the area. Often this coring material is end grain balsa. Balsa is chosen because of some strength advantages over foam and sometimes due to cost reasons as well. Unfortunately, balsa is not forving to shoddy construction techniques, which quite frankly are in fact the norm, not the exception in the marine industry. Wellcraft over the years has built some good boats but like other builders has not been beyond question on build techniques. Nor is balsa forgiving to leaky fasteners, hatches and any other potential source of water intrustion. I'm being nice by not calling into question what owners/boatyards may have done on the boat since new. While its mostly a myth of balsa wicking immediately across its entire surface (that's why they use end grain), often it was installed and bedded poorly and if the problem has been allowed to fester for a long time, the entire core can be suspect. If you catch it early though, the problem CAN be isolated within a defined area and dealt with accordingly.
So....here are the items you need to consider.
1.) Is the area in fact cored at all? Usually you can pull a fasterner or two to discover this.
2.) If it is cored, what is it cored with?
3.) Has the core seen water intrusion? Again, often by pulling fasteners you can discover this. What you don't want to discover is dark discolored or black mushy. Either is a problem or potential problem.
If your surface is not cored, its likely just a thiner part of construction used to keep the overall boat weight down. There is a big distinction between the cabin roof and location of a possible hatch, and the bow area where cleats/windless would be mounted. I'm not familiar with your boat, but don't accept flex in an area of high stress. The tension on a windless or bow cleat is enough to tear the area right out of the boat.
If the area is cored, I would be highly suspect of a compromised core. This is a VERY common scenario. Depending upon the boat, there sometimes is a silver lining to a failed core in this area of a boat. Namely if you can get easy access to the interior of the area, core can replaced from the inside, where cosmetically the fix will ultimately not be seen. Often these fixes are not as difficult as they seem, IF, you can get access.
Your next question, not asked yet, will be whether you can just let it go? The answer is NO if it is around any areas of needed strength such as around a winless, cleats, etc... If however its just a cabin roof, you will need to stabilize the area and ensure it does not get worse. There is nothing on this planet you can smooge though any glorified straw to "fix" what is in there. However, there are some products that can be smooged through a straw to keep the area from getting worse, BUT, they only work if you can dry the area first. Nothing will work against wet material. Of course the best solution to drying the area, drum roll please....is to open the area up. Once you have done that you have done 80% of the hard work to replacing the core anyway. There are methods by which you can drill a series of holes from the inside to help the drying process, which sometimes can work. They are rarely employed by glass workers as in the final analysis because it in fact takes longer to go that route than just opening up the patient and performing the needed surgery.
So....I think your next step is to take the above summary and go back to the boat and do some more investigation. Do you have a core?
BTW, one last observation. I noted you used the language "resealed around.....". that leaves me with the impression that you have noted leaking rail stantions and hatch. It also leaves me with the impression that your solution to rebedding those items was to simply try to caulk around those items. If I'm right, then I'll state that you not only did not make anything better, but also made an unsightly mess around them as well. Bottom line is this, unless you remove those fittings and "bed" them with a good marine grade sealant, preferably not silicone IMHO then you are wasting your time and just creating a bigger problem to solve. Sealant needs to go between clean mating surfaces. Any other approach just doesn't work. The good news is that when you bed something properly you hardly see the sealant at all and it will last many years if the area is structurally sound.
Good Luck.
