At least the image scaling does not appear to be in linear? Going by the notes in [0]:

    Gimp and stb_image_resize results are subtly different from others, even for Bilinear. Looks like they do convert source sRGB image to linear, do filtering there, and convert back to sRGB.

You can definitely see that as a problem with bright lines (e.g. collar highlight in einar*.png) being darker in the upscaled Blender versions.

[0]: https://aras-p.info/img/misc/upsample_filter_comp_2024/