Sorry, I can't agree that there's some unspoken promise, or even a faint expectation, that that the central bank will buy its own currency back. This is simply not true.
Buy back is implicit in the price stability mission any central bank has; it cannot be fulfilled without controlling the supply of currency in the economy, i.e (at least) limited buyback. Without that, what you get is something very much like a cryptocurrency in terms of volatility.
There is no implicit promise of buying back their own coins. This is ridiculous.