I strongly suspect that given that this has happened to two of them, it is how you are pulling at it. I find that if I pull upwards, its hard as hell to open, but if I pull perpendicularly forwards, towards me, it unpops easily.
I noticed this early on when it was frustrating to open at times, until I twigged to the "pulling it towards me" with no upward angle involved.
I just retried this with my bag while writing this, and it is definitely much, much easier to pop them and after a short time owning it, this changed my initial view of the bag (that it was harder to open)
I had had a zipper bar bag before, which I initially missed because it was easier to open with one hand to grab a camera while riding. Now I see how to open the Ortliebs this way, Im okay with it.

overall , I have always been impressed by the longevity of Ortlieb bags, and they have been making this bag for ages, same snaps, so unless some odd batch has come along, I really do wonder that you have been forcing it hard the way described.