I had a straight.el issue with plantuml-mode, using a fork of this package resulted in this warning:
Warning (straight): Two different recipes given for “plantuml-mode” (…)
flycheck-plantuml has a dependency (“Package-Requires”) on plantuml-mode. use-package’s :after appears to be powerless in this case:
(use-package flycheck-plantuml :after plantuml-mode
(use-package plantuml-mode)
Solution (see link): swap the use-package calls, the order matters.
https://www.reddit.com/r/emacs/comments/z836u9/i_do_not_find_the_origin_of_this_error_can/