So you've discovered the difference between "metallic" and "non-metallic" locking nuts (commonly called Nylock). In the aircraft industry both are considered a single use fastener. The difference is the corresponding bolt is more likely able to be reused with non-metallic locking nuts (whether any fasteners should be reused is another discussion). Metallic locking nuts tend to tear up the threads on really good bolts of similar hardness, I can only imagine what one would do to a softer thread.
Best practice is to do any mock-up with standard nuts and swap to locking nuts after final assembly when the fasteners are torqued/cotter pinned/safety wired for the last time.
Perhaps PST could add a note to their instruction sheet recommending standard Grade 5 nuts for mock-up, and for a few pennies, throw a couple in their kits. And, although not visually correct Grade 5 Nylocks would be plenty adequate for strut rods, either end, allowing for easier future disassembly.
