TPS60312 charge pump severely drops zinc-air coin cell voltage at higher output currents
You are severely overloading that cell.  Its 600 mAh rating is at a 2 mA current draw.  To get your desired 20 mA at 3.3 volts, you will draw at least 47 mA from the cell, assuming 100% efficiency of your charge pump (and it definitely won't be 100% efficient, so will draw even more current.)