The Federal Reserve's plan to begin unwinding its unprecedented backstop of corporate debt is rekindling an idea that many have warned about: that investors are now convinced the central bank will bail them out again if needed.

From Neuberger Berman to Invesco Ltd., investors say that the Fed's intervention at the depths of the Covid-19 pandemic provides a model to follow for future crises, which isn't necessarily what the central bank wanted to communicate.

Chairman Jerome Powell has said the Fed would act as a backstop only in once-in-a-generation type emergencies. Yet risk premiums barely moved after the central bank said it's going to gradually shed those investments, and companies are still selling bonds after a relentless rally over the past 14 months that's driven borrowing costs to all-time lows and debt issuance to record highs.