/var/www/discourse$ bundle exec rails c
Loading production environment (Rails 7.0.7)
irb(main):001:0> Post.update_all(cook_method: Post.cook_methods[:regular])
=> 13809
irb(main):002:0>
Done! Sadly no change.
But I solved it. And I am sorry for not mentioning what plugins I had installed as they can interfere…
I have the Topic Ratings Plugin installed and enabled for the category into which my imports go.
On a whim I disabled the plugin and tried again to edit a “broken” post. Now upon saving, the Markdown formatting was used for fancy rendering. I re-enabled the plugin and tried again with success. I imported new posts and it also renders the posts correctly now (using cook_method: Post.cook_methods[:regular]).
I have no idea what is/was going on there but it seems to have been fixed by toggling the Topic Ratings Plugin off and on again.
Thank you all for the great suggestions and problem hunting!