AppStream metadata for the tuxpaint package are not generated: https://appstream.debian.org/sid/main/metainfo/tuxpaint.html https://appstream.debian.org/trixie/main/metainfo/tuxpaint.html This seems to be caused by the desktop icon being too small: https://appstream.debian.org/trixie/main/issues/tuxpaint.html "Only a very small icon ('tuxpaint_tuxpaint.png', 40x40px) could be located. Please try to supply a bigger icon (at least 64x64px) in the Freedesktop icon path (/usr/share/icons/hicolor/%size%/apps/)."
This causes Tux Paint to be missing in modern GUI software centers such as GNOME Software or KDE Discover, making its installation very hard for regular users. Please, consider fixing this issue by providing a better desktop icon (or at least an upscaled 64px x 64px version of the original one).