This is the market signaling that it no longer perceives any value in the maturity value of these bonds. They aren't being purchased for their maturity value; they're liquid assets, traded like any other in a market awash with ultra wealthy institutional investors between whom these securities flow like any other asset.
Isn't inflation simply another cost? As long as the value of these bonds (their low risk, as opposed to their maturity value) is greater than whatever cost you care to consider (inflation, negative interest, opportunity cost, etc.) they will be valued instruments. There is no "what should happen" or what is "supposed" to happen; those are fictions in the minds of spectators and until you're prepared to anger some powerful people and institutions they will remain fictions.
> My gut response is that, yes, if you’re buying risk-free treasuries, why should you get a return above inflation at all? Rewards and risks should be commensurate.
The parent has some sort of ad hoc economic theory ("gut response"), and I am asking them to expand on what the theory entails. "Supposed to happen" means it would be predicted by this theory of theirs.