This is a weird sitch. When the outside temperature is high, the heat pump should have *less* difficulty pumping heat into the car, unless the software won't let it run above a certain outside temperature (which I guess is what you are suggesting by reading the manual for an explanation).
Maybe try running it in defog mode, with the heat and AC both on? I'm not even sure how that works (pathetic noob who needs to read the manual), but it seems like what it *should* be doing is running the air through the cold (AC) coil to dehumidify it, then through the hot coil to warm it (so BOTH sides of the heat pump system are working on air bound for the cabin, not the usual where one is outside exchanging heat with the exterior), but if the system doesn't route that way, it might use the cold heat pump coil (AC mode) followed by the resistance heater, which would be a real energy hog. When I turn on my defogger, I smell something like a hot resistance element, so I'm betting on the latter mode of operation.
2013 SV no QC, built July 2013
car grew up in San Jose CA, purchased 5/31/17 in Seattle
on 6/16/17: AHr = 56.4; SOH = 86%; Hx = 84.3; ODO = 39,250
bar 12 lost 8/21/17
on 8/23/18: AHr = 54.6; SOH = 83.5%; Hx = 78.2; ODO = 49,746