I experienced vertical blade play aka lock rock on quite a few of my Striders. Some were bad enough that the lock failed entirely and others were minor like yours sounds. Could not tell you why they seem prone to issues, but as some have mentioned it is a simple fix if you are willing to take a small risk.
For the couple I still own, I disassembled the knife (get a proper pivot tool and if it does not budge, put the knife in a plastic bag or two, soak in boiling water for a few minutes to break down the Loctite, and it will loosen very easily) and added a bit of tension to the lockbar. Voila - no more lock rock and I get the bank vault kind of lockup I expect from a CRK, Calavera, Hinderer, etc.
You might have some lock stick in the beginning so do not overdo it, or you will wreck your thumb. It does not take much to get them locking up as they should. Please keep in mind the potential risk you take servicing it yourself, but frankly it is ridiculous what they charge for servicing/shipping when the knife should be good to go in the first place.
The design, ergonomics, heat treat, etc is all there but something about their lockup is missing. I have returned more Striders than any other brand but I still love the ones I have.