Stablecoins are a house of cards built around stably relating to another house of cards which is the entire inflationary fiat system. Every single asset and currency is speculated on via the open market. Bitcoin is no exception. If it is overvalued or undervalued, that creates market opportunities for people to exploit the difference. The market has decided it's worth a certain amount today, it will be another amount tomorrow. Not unique to Bitcoin. Every year people have said Bitcoin was "overvalued" and powered purely by hype, on average, the market has decided they were wrong the following year.
Any honestly-run stablecoin inherently has to collateralize their coin with something. They can buy BTC (and do), they can buy USD (and do), they can buy wheat futures (but I'm not sure they do). Ultimately, a diverse portfolio would probably be wisest. Yet you don't see anybody complaining that "USD is being pumped by Tether/USDC". Why? Because it's not a problem.