has anybody else noticed this about flac audio?

Guitarsam - Some years ago I made tests comparing same track/file with different levels of compression and comparing to AIFF format. I did not get any difference. That is a fact for my equipment and ears as I am sure that you have really heard differences.I made this because in old posts and reviews there were a strong advice to use AIFF / WAV formats.
This is my opinion: I think the quality of digital music regarding the “computer” where the player runs depends strongly on the “silence” of the hardware: RAM, hard disk, processor usage, power supply, USB cards, etc.
I think that your computer is old and could be “overcharged” with normal operation
A lot of tweaks can be made and if you look into the dedicated hardware to play / serve music to DAC, you will realize it is always based in these tweaks adding in most cases a very “light” operating system that does not charge the processor (Aurender, Innuos, Antipodes, 432evo, ...)
There is also a lot of literature regarding that playing from RAM increases the sound quality: you can find it easily (forums about Euphony operating system are a good example as they recommend this)There are also places where you can find ways to make your computer less noisy but I do not know anyone for Windows.
I have been thru all this for some years and I am slowly increasing the sound quality when using my Mac mini as source.
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I am embroiled with someone on Facebook that says there are differences between FLAC, WAV and AIFF. I say no, and opined that if he did hear a difference it was because he had to in order to fly his audio cognoscenti flag high, and to justify his expensive equipment. He said I couldn’t hear a difference because the Signal to Noise ratio of my Schitt DAC wasn’t good enough. I gave up on the blockhead.

I enthusiastically second everything in @mapman 's post.

At some point, I compared AIFF, WAV, FLAC (even a few levels), and a format or two more. I thought I could relatively consistently notice difference between DSD and others except 24/192 PCM (like from DVD-Audio). It was not a blind test of any sorts, but just a casual comparison in some spare time. I could not discern between AIFF, WAV, and any of the FLACs with any consistency. There were differences, but I could not be sure next time around so it is not worth mentioning.

What I did, and still do, notice is that CD always sounds "better" in its physical format when compared to the FLAC rip of itself played from a dedicated music server via the same DAC.

I fixed that problem by not playing CDs anymore so I do not compare anymore. The difference, if it actually exists in reality, is small enough that, without comparison, I do not feel I am missing anything while convenience is incomparably on the side of rips.

Older vinyl records do sound different to me, at least most of the time, but is it better depends on how much you like that particular "sound signature". I happen to like it on some of the days and cannot care less about it on other days. It may be my hardware, but vinyl transferred into a digital format never sounds the same as the original. But, it fits in a Walkman, SD card, and wherever else, while not being horrible. What is there not to like?

Digital has come a long way in a relatively short time. Older digital isn't anywhere as good as what we are offered now. My favourite recent example is The Beatle's remixes. It is possibly not the file compression but the original source.