Well, after a few days and some contact with customer service, I am assuming this is "normal" operation for the unit (my conclusion, not CS). In the days up to when I first noticed my unit went into low power mode, it was moderately overcast and solar irradiance was down about 25-50% (based on my home PV production). I didn't think anything of it as when TS Hilary passed through So CA last month, it was down by about 50-75% for a day and a half and I noticed no issue with the Betta.
My unit had been working normally (lasting through the night) the past couple days and then there was another moderately overcast day and again the unit went into low power mode shortly after sunset that day and only returned to normal mode about 2 hrs after sunrise the next morning. This leads me to believe that the unit's PV production wasn't meeting demand.
However, I did have some questions that CS did not answer. I was told to take it out of the pool, leave it on and let it charge for a few hours and that a fully charged unit will last 24-36 hrs. I did so and noticed the propellers and paddle wheel continued to turn. I asked the CS how this would help it charge any quicker than leaving it in the pool, since it continues to run, it must be using energy. I didn't get an answer to that.
I also asked how I would know when the unit was fully charged and the CS told me the LED would be flashing green. I told him the unit does that anytime it is on and not in low power mode, and it can't be 100% charged at all times. I didn't get a response to that.
So, I am still wondering if the battery is prematurely aging and not holding charge long enough, if the solar charging system is not working optimally (since I have no way of determining if the battery is ever being fully charged) or this is just the way this unit works in overcast/low solar conditions. If the latter, it's going to suck this fall when my neighbor's ash begins it "shed my leaves directly into your pool" season.