DISCUSSION: "It only comes out at night". Does anyone else have this experience!?

I am guessing that means you don’t have one. If you did, you’d probably believe what the documentation says, like the specs on the all equipment you own. That, and this was published before 24/7/365 gaslighting became predominant (mainstream).

I  supply part of  my system with lithium ion battery pack that puts out pure sine wave. Using on dac compared with AC fed from modded BPT 3.5 transformer based conditioner, I prefer feed from BPT, slightly more closed in veiled sq with batteries.

My take is the inverters in these products affect sq to some extent, YMMV.

There's an easy way to test and see if your system really does what it says it does. If it is truly isolated then it will disconnect from AC and run entirely off batteries. When this happens you will hear the sound improve, and because it is running off batteries and truly isolated it will sound the same regardless of the time of day or anything else. You already said this is not the case so you yourself know it is not isolated as claimed.

Another way I know is another member with the same setup experienced the same thing. 

Third way I know is from personal experience with battery isolation in my own system. It is easy to hear the improvement when going completely off AC and running off battery power. It is also almost as easy to hear the degradation when running off battery power but still connected to AC via the charger.

What happens is really no different than what happens in the power supply of every component. Theoretically, these are all "isolated" by power supply caps. Lots and lots of manufacturers claim, and lots of audiophiles believe, that enough power supply filter caps means nothing upstream from this can matter. Power supply transformers after all are transformers. If you know how a transformer works, there are two coils, primary and secondary, with no physical connection between them. This does work to effectively filter out some of the noise riding on the AC line. Some, but not all.

Reality is that as long as there is any connection at all, including even through a transformer, then AC line noise will get through. This is why you hear the sound change even though if your system worked as claimed it would be perfect isolation and sound the same 24/7. 

There is no guessing involved. This is not a case of what I "believe" to be the case. This is a case of what I know to be the case three different ways: personal experience, others experience, and a solid understanding of the subject.

Interestingly, your own experience corroborates everything I'm saying. 

all my equipment is behind a UPS that completely isolates AC output from AC input by reconstituting the AC voltage and sine wave from a bank of DC batteries.

But, I still notice it.

So where exactly does gaslighting come into it?

It also could be all of the other things already discussed, and the other things unknown and not discussed. It could even be not true and merely subjective perception. 

it could be that the sun bombards earth with far more than light and warmth, and when sufficiently beyond the horizon everything known and that unknown could be it, if it even does exist. And an interesting derivative in language is, that any word that abstractly equates to “may be”, “could be”, creates an inference of may NOT be, or could NOT be, otherwise descriptives such as ABSOLUTELY would be used.

Re: “gaslighting”, that is in regards to disbelieving anything because it is said, “the person selling it says it works”, which in extreme gaslighting paranoia would require that whatever is said must be disbelieved but for the fact it was said. Or, because it was said (sold) it mandates it be disbelieved.

such as, what was just said.

if it is said, it must be disbelieved.

WHICH is in large part is going on in our present day “media”, mainstream. You can’t believe what you see, 24/7/365, which is gaslighting.