ICC profile being ignored???

I have only noticed this in the past few days, and it took me a while to figure out what the hell was going on.
Basically anytime I'd got to Settings-display, the monitor would change colours. It turned out the freaking ICC profile I created using a calibration device was not being applied at all despite being selected in settings! What the hell is this? Apparently it's been like that forever. I do photography as a hobby and need realistic colours for this, and now I fear anything I did in Windows 11 was junk, because this smells like something that's been here forever rather than being a new bug.
I am on 25H2 btw.

Does anyone know anything about this?

Windows ICC profile loading is flaky. I use DisplayCAL Profile Loader which is free.

I use Calman and their "Client 3" program to manage the ICC Profiles they create. They explained to me a long time ago that Calman adds a video card 1D LUT to the profile, and Windows wipes it out when I go to Display settings. Moreover, Windows doesn't apply it when I select a Calman profile, so I have to use Client 3 to reapply it. Of course, the advantage to the Calman profile is that the 1D LUT is system-wide and affects apps that aren't color-managed. It's a minor annoyance and mostly stable outside of Display>Settings, which actually has gotten better over the years about this. However, the Calman people also told me any program (in particular, games) can blow up the 1D LUT, and they get no notification of it, so you just have to deal with it.

I know for a fact this worked reliably in Windows 10, and when I switched to Windows 11 I did the same thing (calibration), not expecting it to behave any differently, and forgot about it. It wasn't until these days when I purely accidentally noticed screen changing colours when I open the relevant display settings page. This cannot be working as intended, it really seems like the profile which is selected is not being applied at bloody all.

If by "screen" you're including the desktop background, the desktop isn't color managed (I guess ACM could be a factor, but I disable it so wouldn't know), so it sounds like the Calman Client 3 situation I described. In fact, watching the desktop do its slight color shift is something I always watch for when I reboot and Client 3 runs. The problem with Display>Settings long predates Windows 11, which actually has gotten a little better about it over the last year or two for me. I started using Client 3 in 2014, so that would've been Windows 7 or 10. (I skipped Windows 8.) What software are you using?

What do you mean by colour managed? I thought the ICC profiles apply to absolutely everything since it's system setting, so it should adjust colours of everything on the monitor.

What do you mean by what software am I using? For calibration? Calibrite.

No, ICC profiles traditionally have only worked with "color aware" or "color managed" applications like web browsers and various paint programs and image viewers, some of which provide settings to disable the feature. My understanding is that Windows 11 ACM tries to make it more system-wide, but like I said, I disable it - it relies on things like EDID accurately providing the information it uses, and ACM is NOT calibration. I use actual calibration with a meter and software, with Calman Client 3 the ICC manager, and as previously explained, Calman adds a video card 1D LUT that isn't a complete color profile but still tunes up grayscale and gamma system-wide to make an easily noticeable improvement.

I don't know anything about Calibrite, but what you described is what I've observed with Calman and its Client 3 since 2014. It may be due to the same thing. It may be ACM injecting some variability.

But the thing is, when I open the old Control panel and go to advanced tab of colour profiles, there is a button called "reload current calibrations".
When the ICC profile is unloaded - BTW this also happens when you put the OS to sleep - clicking on the button changes colours on ALL opened windows, including for example Thunderbird, which based on your post should not be affected by ICC profile, so I am really confused.

edit: I have ACM disabled, and ironically, when I enable it, the calibration is "lost" upon waking up just as well. Damn this bugged OS to hell and back.

Side question:
When I open the Settings and click on start menu or anything that takes the Settings windows out of focus, it completely changes colour temperature from reddish to something noticeably cooler. Is this normal? Does that happen for others too?
The temperature looks exactly as if the ICC profile was not applied and the default builtin sRGB one was being used instead (when in focus), so it looks kinda suspicious to me.

Color calibration makes some difference if you are using onboard graphics. I noticed that my nVidia card makes very little difference with the default settings and color calibration, so I don't use it in my main PC (2nd system specs), only on my laptop with Intel graphics. I also noticed that each time I connect my laptop to the TV via HDMI the color profile is supposedly used, but the colors are not perfect. I have saved a color profile in Intel Graphics Settings called "Default" and when I select it and apply it the colors are well calibrated. So you might do the same. You don't need to make any changes to the color from Intel (or whatever you have), just save a color profile with the settings after calibration. Then every time you see the colors off, just load the profile and they should fix.

PS: I have this bug since Windows 10, it is not a Windows 11 25H2 bug.