That's your problem right there. The HLG driver recommendations are for LED panels run in series, not parallel.
Running in parallel means you are only using 56V or less and the current is divided between each board. When you run in series you double the voltage but get full current to each board.
Wire them in series and report back.
And in future, if you want to run in parallel, use a constant voltage driver like you had originally – HLG-320H-54V for example – or re-wire your boards in series. That HLG-320H-C2100B is not designed to power 56V LEDs as the constant current region is 76-152V, so when they are wired in series (56V + 56V = 112V) they will be right where you want them to be.
BTW, 112V x 2.1A = 235W (series) whereas 56V x 2.1A = 117W (parallel). Those QB648s won't be drawing 56V at such a low current, so I'd guess you are probably only drawing 80-100W from the wall. So yes, you are getting less power from that 320W drvier than from your original 150W driver.