I think I see what the issue is
I've been playing around with my screen resolution, and it's set at the next to the lowest image size setting (so next to highest resolution). If I increase the image size, by making the monitor, which has a 2560 X 1440 max resolution, behave like a 1920 X 1080 monitor, I can see the diagonals at 100%, just about when looking quite close. I'm talking about the image of the cat with his tongue out, looking at his whiskers. I tried a couple of different resize methods, and I think the bilinear method on PL6 gives slightly smoother results than the default, which is bicubic sharper. I also processed the image without resizing, and don't see the diagonals on that one. Here's the non-resized image (it is cropped a bit):

And here's the original version I posted, which is bicubic sharper resized to 2000 pixels on the longer side:

And here's the same one, resized with bilinear:

I can't see how that looks at 100% until I post this, and then look at the images on DPR's site. So I'll come back in a minute.
I've just had a look at 100%, and I can definitely see a difference between the original bicubic sharper resize and the bicubic linear resize. If I set my monitor to behave like a 1920 X 1080 monitor, I can see some diagonals at 100%, more so on the bicubic sharper version, but not on the original (non-resized) size version. At the default resolution, which is 2240 X 1260, I don't see them, and not at the monitor's maximum resolution either. It looks like some resolution settings are showing the images at 100% as if they were really more like 150% or 200%. It also looks like the resizing algorithm is what is introducing the diagonals, which are only visible at certain screen resolutions, at least on my iMac.
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