Lol, I have a BL2420PT too.
I can agree that it is NOT a good monitor, the main issue I have with mine is that it 100% has FRC temporal dithering.
When connecting any input, even a super old Windows computer, by moving my head left to right I can make out an alternating intense red and blue checkerboard pattern that keeps alternating left and right for the first few minutes after turning on the monitor. This happens on all inputs, HDMI, DVI, and DP.
Then, the more aggressive checkerboard pattern begins to """stabilize""" (but not actually truly stabilize…) into another dithering pattern that looks more like random moving static noise.
If I put a dark gray rectangle on the screen, I can literally see moving static in the rectangle with my own eyes while looking close up at the screen. This is especially easy to spot when using max brightness and messing around with color settings to make dark colors look brighter.
My BL2420PT is definitely 6-bit + FRC despite no one stating this online anywhere, because if I connect it to an M1 Mac, I can visibly see Apple's temporal dithering pattern clashing with the monitor's own. If I then activate Stillcolor, Apple's dithering stops, but I can still see static coming from the monitor itself especially on darker grays.
The amount and pattern of moving static will actually differ between each unique shade of dark shade of gray that is on screen.
The color settings on the monitor are also really confusing, it's not clear what "Gamma" mode is true linear gamma.
In addition, there's this really weird quirk where the monitor always applies a warm tint to it after booting up no matter what color settings you use.
If you have "Custom" color temperature set with R, G, and B all set to 255 (the only way to get anything close to native, as according to the Factory menu ALL preset color modes including "sRGB" do not actually use full or equal RGB color primaries…)
You actually have to enter that menu, do nothing, then press back and then it will suddenly switch to a more cold-looking image. I have to do this every single time I turn on the monitor to actually get natural colors.
In my case, watching videos or playing games on the monitor (AKA, using it as a TV) is totally fine and that's when it actually looks pretty crisp, but the moment I'm actually using it for computer use it just immediately makes me tired to work on. It's just really hard to focus on text without suddenly feeling really fatigued. Background colors also look really unstable.
I don't have it at the current location I'm living at right now, but for some reason I also kind of remember seeing "SLA1" when checking out the service page a few months ago? Might be misremembering though.
I wonder if I also have the worse panel that you have. I bought mine all the way back in 2016. My main issue with mine seems to be FRC and not PWM though.
The size + resolution combo is also just bad in general, especially on macOS you only get "extremely small 2K" and "extremely oversized 720p" as your only pixel-perfect options.
Due to this, years before I learned about PWM or temporal dithering I was already having strain with this monitor but always had (mostly incorrectly) chalked it up to size+resolution being the reason.
Since 2017 it just never felt like I could get any actual work done on this monitor, I remember trying to code on it a bunch of times and then either just getting really tired or distracted (pretty much due to not being able to read) or switching to my laptop instead only minutes into using it.
I would always end up constantly adjusting the brightness to try to make out text, and was switching back and forth from the BL2420PT to older TN monitors constantly (which weren't much better in retrospect, as they also all have FRC) and at that point I had no idea why.
I didn't truly realize my reading comprehension issues were literally coming from the screens I used until 2022, and didn't even fully understand what temporal dithering was until 2023…
I unfortunately currently own no external monitor without FRC, so for now I switched to an older 1080p Samsung TN monitor that also has some FRC but a lot more subtle, no PWM at max brightness (but REALLY agressive PWM anywhere below that… LOL) and most importantly a much more comfortable feeling backlight spectrum for my eyes.
I just end up using my laptops for now though, possibly due to lacking an actual FRC-free monitor, but mostly just because I hate sitting in one place at a desk forever anyway (external keyboards just don't work well ergonomically for me).