Color reduction inconsistency
When pasting a 24-bit RGB image into an indexed color document, GIMP uses a specific color reduction algorithm and doesn't allow any choice. However, that color reduction algorithm is unavailable when converting an image from RGB to indexed mode "in place".
This is related to issue 1498 and I will be using the attachment from there to demonstrate. I will first select the entire image and copy the contents to clipboard. Then I reduce the colors with Image>Mode>Indexed, picking "Use web-optimized palette" and selecting "None" under dithering options. This gives me the following result: Now I just paste the original from the clipboard into the now-indexed document. This gives entirely different result:
To me the latter seems obviously superior, but both could co-exist as options in the conversion dialog (possibly with an alternative perceptual algorithm?).
Similar-looking "torn edges" can be seen with positional dithering as well, so this likely affects all methods, not just the non-dithered one.
(GIMP 2.10.18, Windows)