Inflation wasn't an issue for the Native Americans because...
Wampum wasn't really currency to the native Americans - they valued it, but were not making it with the intention of it being used like coin. Positions within tribes would be marked by the wearing of wampum necklaces and those pieces would he passed to the next individual to hold that station. It represented prestige and carried a symbolic, spiritual message. They were given at births, deaths, as invitations, during peace treaties, and as a means of communicating with the dead.
It was difficult to manufacture. Picking up a quahog shell in the summer didn't guarantee you would have a finished product of wampum. Cutting the shells and drilling holes with hand tools was very difficult - shells have a tendency to be chalky and brittle and crumble under too much pressure. The value in the wampum was the difficulty in producing it - and the rareness of certain colors, like black wampum.
When inflation and counterfeit started to be seen with eampum, it was because the Dutch saw the potential for making a fortune by manufacturing it and trading with Native Americans - they were already using venetian glass beads as currency in Africa and India. They forced the manufacture of wampum, stringing it in terms of monetary value ("1 belt of white wampum would have an exchange rate of x amount") rather than the symbolic stringing the natives used. Wampum was adopted (hesitantly at first by Native Americans) as a currency and set valuations were put in place. Fake wampum was produced, just like counterfeit money was a common thing - rare black beads were faked by dying white ones. Within 30 years, wampum went from a prestigious item to a set currency to having its set valuation removed (so that it had no monetary value beyond what was agreed by individuals involved in the barter) as the fur trade slowed.