I'm having the same problem, basj.
I'm using Windows 10 Version 21H2 (OS Build 19044.1469)
IconPackager Version 10.03
And it happens mostly with the older packs. The ones designed for Vista. But I thought they would still skin properly for Windows 10. My problem is the same as the OP. The icons will display properly when they are set to small but will revert back to the normal Windows 10 icons as soon as you try to resize to anything larger.