I just had another failure last night. Went to top off my tires and couldn't on one of them. Changing the valve fixed the problem.

I've certainly seen more bad presta valves in the last few years than I've seen my entire life. We know that not all low-cost things produced in China are junk, but some of them certainly are. This looks like an example.

I mentioned above how much better the replacements from Amazon are than what I've pulled out of tubes lately. I looked at them side by side under a low-power stereo microscope (because I could) but even with just a strong magnifying glass the difference in quality is apparent. Both the materials and the fit and finish are significantly better on the replacements.

It's a shame that now I have to just assume any new tube will come with a junk valve, even Schwalbe or Continental tubes, and now just replace them automatically. And now I always carry a spare, which I didn't use to.