Why are my stir fried pea sprouts too stiff, and turn into lumps?
I don't make the Hong Kong recipe, but I often cook Thai-style pea shoots in the spring.  And for that matter I often made them when I lived in California, so origin is not your problem.
One thing I did find is that there's a huge difference in pea shoots based on age. Tiny, 3-day old pea shoots, also called &quot;pea sprouts&quot;, are the kind you want to flash-fry:

Pea shoots that are a bit older, like a week or two old are also eaten:

However, these are a lot more fibrous, and need to be blanched before frying.  I don't know what you bought, but it really sounds like you got older pea shoots.  Species might also make a difference; the ones that Asian markets sell are usually snow pea shoots.  If you got some other kind of pea shoot, like English peas, it's possible that those are also more fibrous.