The issue is investment, not consumption. When deciding what to invest in, expected returns are calculated net of taxes, inflation, and risk. Capital generally flows to the investment with the highest expected net return at every risk level.
In a deflationary environment, sitting on investable cash grows risk-free and tax-free, which makes it an attractive "investment" for many category of investor instead of putting that capital to work.
Isn't that a self-correcting problem? If the money supply were held constant, and people didn't invest, then production efficiency would not improve, and there won't be deflation, right?
Even for "risk-free" assets like cash/bonds, inflation risk always exists. It's essentially a risk that you don't have a counterparty willing to trade the things you want.
In a deflationary environment, sitting on investable cash grows risk-free and tax-free, which makes it an attractive "investment" for many category of investor instead of putting that capital to work.