With k9 mail 8.0 (android 14,pixel 6a) no stars appear in the message listing view. They only show up in the full message display. Since the main function of the star flag is to make it easy to find messages that need attention, they need to be visible in the listing. This seems to be a new bug in 8.0.
Note that stars are present in the full message display, and show up in list view on outlook app and thunderbird on desktop pc. Also, if I select a message in the k9 list view and bring up the 3 dot menu it offers options of either “remove star” or “add star” depending on whether the message has one.