FFMS2 changing RGB values when importing palette-indexed BMP/PNG files

[SilSinn9801]

I don’t know if this is a bug in VapourSynth R63 or in FFMS2 3.0.1.0 1325+16 6ad7738, but basically when importing (via core.ffms2.Source) indexed BMP or PNG images (that is, images whose pixels don’t store RGB color values but rather indices to a color palette stored in the files’ metadata), some of their pixels’ RGB color values get changed to slightly-different values, even when they’re imported in RGB24 mode (rather than a YUV or YCbCr mode). (This problem does not happen at all with regular RGB24 BMP/PNG files where RGB values are stored directly in the pixels themselves instead of in a palette.)
Attached to this thread is a ZIP with two visually-identical 16×16-pixel BMP files where each pixel is a different shade of gray (out of the 256 values allowed in GRAY8). Y8_index.bmp is the indexed or palettized version (8-bit grayscale palette, 256 indices), while Y8_rgb.bmp is the RGB24-mode version. This is the VPY script I am running to test VapourSynth & FFMS2’s behavior when handling indexed images vs. RGB24 images:

import vapoursynth as vs
from vapoursynth import core

Y8_index = core.ffms2.Source(r'Y8_index.bmp',fpsnum=60,fpsden=1)
Y8_rgb = core.ffms2.Source(r'Y8_rgb.bmp',fpsnum=60,fpsden=1)

FinalVideo = Y8_index*60+Y8_rgb*60
FinalVideo.set_output()

Using VirtualDub2 build 44282 as the VapourSynth visualizer (& Python 3.11), I see that the following 36 RGB values from Y8_index get changed to the following (darker or lighter) values at the output:

• #040404 → #030303 (darker)
• #0b0b0b → #0a0a0a ("")
• #121212 → #111111
• #191919 → #181818
• #202020 → #1f1f1f
• #272727 → #262626
• #2e2e2e → #2f2f2f (lighter)
• #353535 → #363636 ("")
• #3c3c3c → #3d3d3d
• #434343 → #444444
• #4a4a4a → #4b4b4b
• #515151 → #525252
• #595959 → #585858 (darker)
• #606060 → #5f5f5f ("")
• #676767 → #666666
• #6e6e6e → #6d6d6d
• #757575 → #747474
• #7c7c7c → #7b7b7b
• #838383 → #848484 (lighter)
• #8a8a8a → #8b8b8b ("")
• #919191 → #929292
• #989898 → #999999
• #9f9f9f → #a0a0a0
• #a6a6a6 → #a7a7a7
• #aeaeae → #adadad (darker)
• #b5b5b5 → #b4b4b4 ("")
• #bcbcbc → #bbbbbb
• #c3c3c3 → #c2c2c2
• #cacaca → #c9c9c9
• #d1d1d1 → #d0d0d0
• #d8d8d8 → #d7d7d7
• #dfdfdf → #e0e0e0 (lighter)
• #e6e6e6 → #e7e7e7 ("")
• #ededed → #eeeeee
• #f4f4f4 → #f5f5f5
• #fbfbfb → #fcfcfc