We also have R710's and have the same issue too with the SG300-10MPP switch.
We're running the latest firmware on the switch (1.4.5.02), have LLDP enabled, have the PoE-PSE Optional TLVs on every port enabled too (with the corresponding LLDP MED Status checkbox enabled).
As a workaround I've been forcing 802.3at by using the set power-mode AT directly on the access point CLI. Once you type the command it will take a few minutes for the status to update. You will notice the power draw stats on the SG300-10MPP will update too.