Repeating characters cause post creation to fail

Hi there. It appears that including certain sets of repeating characters in the text of a new post causes some kind of a problem with WordPress, where I receive a “Post updated” confirmation message, but the content of the post is empty and when you click on the “view post” link, there’s not actually a new post that has been created.

I have been able to reproduce this behavior in the Classic editor on the admin back end with post text like this:

(something something)
×××××××××××

(that’s 11 x’s)

I have also seen it happen with text like this, although only if the underscores are repeated more than one:

(something something)
________________________

(something something)
________________________

(24 underscores)

WordPress doesn’t seem top throw any kind of error message when this happens – it just blanks out the post. Sometimes other odd things happen, like custom fields get duplicated or the category reverts to the default, instead of what I specified when creating the post.

This behavior also shows up with a custom form I created that uses wp_insert_post() to create new posts. In fact that’s where I first discovered it.

Any idea why WordPress seems to be choking on repeating characters like this?

I’m using WordPress 5.5, and plugins:

Advanced Excerpt
Akismet Anti-Spam
Classic Editor
Health Check & Troubleshooting
IntenseDebate
Jetpack
Really Simple SSL
reCaptcha by BestWebSoft
Search Everything
Smart Quote Fixer
Subscribe2
W3 Total Cache
WP Status Notifier
WP to Twitter
WP-Postratings

Of the above, WP Status Notifier might be a culprit since it is tied into status changes (post creation, switching from “draft” to “pending,” etc.). However, deactivating this plugin doesn’t change the behavior.