XnView Software

Windows 10 Pro 21H2 (x64)
XnView MP v1.00 (x64)
G'MIC v3.1.2 (x64)

Example: Image → Adobe Photoshop Plugin… → G'MIC-Qt… → Testing → PhotoComix → Vivid Colors applied to PNG_transparency_demonstration_1.png

Thanks to your detailed description I can reproduce the problem.

This problem is supposed to be fixed in XnView MP 1.01. Please check and confirm the bug fix here.

Works as expected in v1.01 Thanks, Pierre!