Weird category problem (not in list)

Latest WP (6.0.1)
When creating or updating a post a newly created category (created via Posts > Categories) does not show up in list of categories. However, when doing a “quick edit” on the same post the category is in the list and can be selected. After updating that change and then editing the post in the Gutenberg editor the category is visible (and selected).
After de-selecting the category, updating the post, returning to the list of post and opening the post again with “edit” once again the category is no longer in the list.
So it looks like the category is in the category list everywhere except in the editing page. As far as I can tell only the new category is missing, all other previously created categories are working fine.
As a test I created yet another category and that one also is missing from the list on the edit page.

I tried the same on another site I manage and it behaves the same. A category I created a few months ago but haven’t used yet does not show up in the list either so it looks this problem exists for some time.
Both sites use WP 6.0.1, different themes, mostly different plugins.

The problem categories all had no parent, when I put them under a parent they do show up everywhere. Is this a bug or…???

Thanks for any ideas.

• This topic was modified 3 years, 10 months ago by Yui. Reason: moved to fixing wordpress