ristretto fails to open heif image from file system (commandline or Thunar)

Env: XFCE 4.18 (openSUSE LEAP 15.5), ristretto 0.13.1

Packages libheif1, gdk-pixbuf-loader-libheif, heif-thumbnailer are in place and Thunar shows thumbnails of heif files!

Opening email attachements of mime time image/heif fires ristretto!

But opening heif files from Thunar fails. From command line (bash) as well, error message is "Unsupported mime type".

/usr/share/mime/alias contains entries:

image/heic image/heif
image/heic-sequence image/heif
image/heif-sequence image/heif

Why is ristretto failing to render heif image? Bug, missing config?