Hello everyone, I have been having this annoying problem with my Pioneer 12000 BTU 22 SEER inverter mini split ever since I installed it in 2018 and the manufacturer has not been much help in explaining what is going on or resolving it, so I'm hoping someone on here may have some ideas.

The problem is once the outside temperature starts to drop, the unit stops cooling properly. It still continues to run, but no longer cools my house adequately. During the middle of the hottest days we get here (around 95-100 degrees), this unit has always easily kept my living room at around 72 degrees. Where the problems start is in the evening when the outside temperature starts to drop while my attic, brick walls, etc are still heatsoaked from the sun. Once the unit starts to underperform in the evening, my living room gets as warm as 78 degrees or sometimes even higher.

Once the outside temperature drops, the unit seems to slow the compressor down even while the inside temperature is still well above its setpoint. Perhaps something could be going on with the electronic expansion valve not properly adjusting for changing conditions? Not sure.

Here's some data taken at different temperatures with the unit set to 62 degrees (as low as it goes) and "turbo" mode active (evaporator fan forced to full speed)

Outside temp: 96 degrees
Power consumption: ~950 watts
Return air temp: 80 degrees
Supply air temp: 48 degrees
Delta T: 32 degrees

Outside temp: 83 degrees
Power consumption: ~910 watts
Return air temp: 80 degrees
Supply air temp: 54 degrees
Delta T: 26 degrees

Outside temp: 78 degrees
Power consumption: ~440 watts
Return air temp: 82 degrees
Supply air temp: 58 degrees
Delta T: 24 degrees

I just don't understand what is going on here. I don't understand why the unit sometimes loses performance as the outside temp drops without power consumption changing much (indicating the compressor is still working hard) nor do I understand why it seems to slow the compressor down dramatically enough to cause a noticeable loss in performance while the inside temp is still WAY above its setpoint? Maybe some kind of stupid energy saving strategy to cheat the efficiency rating?

I asked Pioneer's support what could cause this several times and no one had any ideas. I asked if this is by design and was told that it is not, but they didn't seem to think there was anything wrong with the unit or its operation either, they said I just undersized it. But if it is so undersized, how does it always perform perfectly during the hottest part of the day and only struggle once the outside temp drops? Makes no sense.

Anyways, any ideas would be greatly appreciated. Thank you very much in advance!