SUMMARY
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PNGs thumbnails can be displayed only for 8-bit PNGs.

Checking my other PNGs that, I created via '$ gm identify exapmle.png" revealed that Dolphin (Plasma 6) cannot display thumbnlails, unless its 8-bit. 12-bit or 16-bit PNGs thumbnails are NOT displayed. Its a regression, because, it used to work in Plasma 5.
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STEPS TO REPRODUCE
0. Check if Dolphing has "no size limit" for thumbnails (its default, but make sure, so you don't ask this question).
1. Install graphics magic to identify PNGs via  $ gm identify
2. Use gimp to create PNGs (8-bit, 12-bi, 16-bit)
3. Open Dolphin to see that only 8-bit PNGs have thumbnails (unlike Plasma/KDE6)

OBSERVED RESULT
Lack of 12-bit or 16-bit PNG thumbnails

EXPECTED RESULT
Make it work as in KDE 5.xxx to display all thumbnails for PNG regardless of bit depth (8-bit sucks and is difficult to compress [wbp, avif, jxl,...] and results in artifacts...).

SOFTWARE/OS VERSIONS
KDE Plasma Version: 6.0.1
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2
Graphics Platform: X11
Graphics Processor: Intel Only

ADDITIONAL INFORMATION
various $ pacman -Qs:
kdesdk-thumbnailers 24.02.0
kdegraphics-thumbnailers 24.02.0
ffmpegthumbs 24.02.0
qt6-imageformats 6.6.2
kimageformats 6.0.0
libpng 1.6.43
libavif 1.0.4 (stil no thumbnails in Dolphin)
libheif 1.17.6
libwebp 1.3.2
webp-pixbuf-loader 0.2.7