arcniz makes an excellent point re cavitation:
Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.
Such conditions may well exist at the HP pump inlet, but the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure. Otherwise we would see backflow, starving the burner and an immediate flameout. I'd don't think that's evident on the DFDR.