Since you mentioned thunar, is this a dualboot with xfce? Sometimes you can have probs with that. It also happens when running it in, say, a wm or another environment. Take a look at, for example: https://discuss.kde.org/t/dolphin-all-file-associations-lost-and-open-with-provides-no-options/12750
But in your case, the entire entry is not there. Hence “does this only happen in dolphin”? Open an image file with Gwenview. Do you have the option there?
