I could be wrong, but I don't think the Microsoft Photos app respects the choices made in IconPackager in either Windows 10 or 11. I tested this by setting Windows Photo Viewer as the default app for .png files and the icon image for .png files in IconPackager was then respected and displayed properly in File Explorer.
If you want your custom icons to display for the different image file types, you could make Windows Photo Viewer the default app for those file types. You'll first need to enable Windows Photo Viewer in the Registry. There are many tech sites on the web with this How-to info. Once you have it enabled, you'll be able to associate your image file types with it. Once this is done, if you want to open an image file in Microsoft Photos, you'll have to open it from within the Photos app, or right click the file and select "open with" Photos.
