Same here (a bit farther north). I don't think it's your heater. I think there are two or three things going on.
Some of the heat we think we are getting from solar is actually coming from the sun directly on the pool. Longer shadows might be having an affect on that source of heat, even though the sun is still shining plenty on our panels.
Sun angle plays a big part in solar heat and electricity generation. The angle is lower now, the heat generation is less.
Nighttime temps play a huge part in the effectiveness of a pool heating system, it's not just about how hot it is during the day.
Solar heating is cumulative over a few days. You add all of the above together, even if each is only contributing to a difference of one or two degrees, it adds up and compounds over a few days. In your case and mine, that's the difference between upper and lower 80s.
My installer's promises about extending pool season a month on each end didn't really pan out. It's more like a couple weeks on each end where I live. I'm grateful for the extra month (total), I know I'm doing better than my neighbor's pool (with no solar), but it's not quite what I had hoped for.
