Can you export the branches

HKEY_CLASSES_ROOT\SystemFileAssociations\.heif
and
HKEY_CLASSES_ROOT\SystemFileAssociations\.heic

and share?

Which version of ImageMagick and Windows? From your description, it seems like magick isn't happening :-) ...as in the magick conversion commands aren't working. For troubleshooting purposes, in the registry entries where magick is called with "cmd.exe /c" you could try temporarily changing it to "cmd.exe /k" instead. This would keep the command window open, and might help with troubleshooting if there is a magick error.