I trust the Ocean and Earth Premium One XT leashes not to break, just cause they've survived several good tugs at the biggest surf I'd go out in locally, and punchier swell on vacation. But twice, in unremarkable surf, I've had the leash cuff pop open. Just now it was a rocky entry, so I know there wasn't sand in the cuff. Is it possible the velcro just starts to loosen up after a year or so [in addition to me not carefully pressing cuff layers together]? One opening event was with a 2-y old leash I use in smaller summer conditions and just now it was a 1-y old leash. Board briefly washed up on rocks but luckily the spot is a low energy novelty wave and I've yet to find a ding on board.
I replaced with a Creatures of Leisure cause the O&E US site doesn't sell 9' ankle leash in XT anymore.
It should be possible to detach the leash from the swivel and attach to a different cuff. I did this to use the xt with my leash lock waist leash I use with my bigger board. Velcro can wear out and like you said keep it clean and press firmly and test. Leash lock is on sale now and they have big wave/sup sizes but I prefer stretchy xt when available check eBay/amazon also for longboard size I think similar thickness to sup on some.
I've had this happen with a different brand about twice recently in gentle waves. Looking at it logically, I believe that it can happen if the leash moves around so that the edge of the cuff faces forward and then a wave manages to perfectly catch the edge and pop the leash open. Luckily it was at a beach break.
[in addition to me not carefully pressing cuff layers together]
I have O&E leashes with many years of use and it never happened to me.
But since I hate when the cuff rotates, I take care to properly close it: finger on the "carpet" part to block it and unroll the "hooks" part progressively, pushing them well together, to avoid having voids between them.
interesting how different people use their stuff..
i close the cuff extra wide that it can rotate around my ankle because i dont like the rope twisted