I posted this in Q&A and got no replies to similar cases.
setup: MP2 5kVA (4.77), 250/100 MPPT, Pylon US3000 (all current GA code)
This is easily reproduced, happens 100% of the time.
If an ESS schedule is enabled (either optimised mode) then the inverter throttles PV for AC loads significantly.
If the schedule is disabled PV returns to normal.
If the mode is changed to "keep batteries charged" it also works fine.
There seems to be a bug related to ESS that is preventing full PV being used.