[Solved] Deleting a post hides the topic from the forum.
The only plugin is "wpForo".
No cache plug-ins, of course.
Since this is a local environment, there is no server-based cache.
We emphasize that
"all cache functions are reproduced in a local environment, even if they are not used."
The one just described is a demonstration, so it is done by a single user, but the actual operation is as follows.
A user who patrols the forum and has no deletion authority "unapproves" and hides posts that are deemed to be in violation of forum rules and etiquette.
After that, the Admin user does a final check and performs a "delete" or "approve" operation.
I tested it in a newly created local development environment of WordPress, with only wpForo as a plugin.
It seems that there is one more condition to be hidden on the forum home page, and that is that the "Category Layout" of the top category must be set to "Extended".
When I deleted a post that had been disapproved under this condition, the problem was reproduced.