Weird. If you’re using the exact same method for changing Frequency and Bar Tempo, and that’s working, that might actually be something besides BPI (Are they the same param type? If not, can you test it with the same param type to see if that’s a factor?). If you can get the wave index to change in other circumstances, I’m guessing it’s not an inherent issue in the MetaSound, either.
Potentially weird suggestion: what happens if you change the name of the input param you’re trying to alter, on the MetaSound’s side? There were some weird pre-release bugs with name collisions and input name propagation, and I can’t think of much else that might be going on.