I think you are still misunderstanding me. The "cracks" you refer to are not cracks per se, they just look like it, but in the reality that part of the cone/diaphragm is completely stiff and rigid and the required movement comes from the (rubber) surround, as in any conventional round-shaped electrodynamic speaker driver.
OK so if I understand correctly the LF driver comes off the production line looking exactly like the one in the OP (even before it is installed in a monitor), with the rough ridges in the diaphragm.
I found this video where at
2:58 we get a glimpse of a bunch of what look like 83
31A racetrack drivers:
A few seconds later we see a close-up of a bigger racetrack drive, probably either 8341A or 83
51
A as it doesn't have the rubber surround with the corrugated sides:
In both of these images the diaphragm doesn't have the rough area, in fact, it looks perfectly smooth all around, unlike the driver in the original post:
Even with my optimistic hat on, I still see torn paper, sorry. If other parts of the perimeter are perfectly smooth, I really don't see why the top and bottom can't also be smooth. I assume the diaphragm is a wet-pressed moulded pulp component. The defining characteristic and the reason for using a mould is that it is able to produce consistently identical parts, but you said the "roughness" is not exactly the same in every drive, therefore the tearing must have happened later, not at the pressing stage. Furthermore, the mould is almost definitely perfectly smooth at that spot anyway, so I'm pretty sure when the diaphragm comes out of the mould the roughness isn't there. I have seen plenty of wet-pressed moulded pulp packaging that has very intricate details and the finish is exceedingly smooth and detailed. The tears therefore must have appeared afterwards, once the diaphragm starts is put through its paces.
I notice the rough area only affects about 80% of the length of the top and bottom parts (respectively) of the diaphragm "bump". The white lines in the above image show where the smooth/rough areas end and begin. Notice also how the angel of the white lines and the sides of the triangular shadow cast by the diaphragm on itself are basically parallel, which suggests a correlation between the tearing and the geometry of the diaphragm. I understand that the rubber surround is there to absorb most of the cyclic stresses so the diaphragm doesn't have to; however, in this case, it appears the diaphragm is in fact absorbing more of those cyclic stresses than it can take, which leads to tearing.
I get what said about accelerated lifetime testing, and I believe it is a reliable way to test if properly implemented, but it's still an imperfect approximation. I used to work in test development myself, so I know a bit about testing. I just have trouble believing the racetrack drivers leave the assembly line with the tears in the diaphragm.
Anyway, I think the best way to settle this and prove me wrong (which I would be infinitely grateful for) is to show me a picture of several brand-new and unused 8351B racetrack drivers side-by-side from the assembly facility. If they have the micro tears like the one in the OP I will have made an ass of myself but I will also be able to put my mind at rest and order a pair of 8351Bs + subs, which was my original intention and the one I would like to go with. If, on the other hand, the brand-new drivers are smooth, then perhaps the product needs reviewing, which is fine by me because these monitors are out of stock everywhere and I will have to wait until spring 2022 to buy anyway.
Regarding the warranty, I will contact support with my suggestions, thanks
Sorry to be such a huge and persistent pain!