My ISP auto-updated all my WP sites to 5 a while back - all without a hitch - except that on one of them I noticed that an CSS control was not rendering on my posts as CSS but as straight text.
A little analysis revealed that Gutenberg had unilaterally decided to add some code prior to that tag, which was causing the problem.
Didn’t happen on the other blogs, so what went wrong?
Calls to the ISP - tickets into WPMUDEV, searching bulletin boards … no luck.
I went in to admin and could edit the post and that was good - but this was going to take a looong time.
Enter Mars Edit.
Synch all posts and search for the offending term - get a list of broken posts …. edit post, save … repeat.
In the end it took me about 30 minutes to modify some 200 posts. A pain in the arse, but waaaaaaay quicker than using the WP admin panel. And I had definitely spent more time than that just researching the problem.
Massive shout out - yet again to @danielpunkass - love that product of yours.
As to what caused the issue? Still no idea. Don’t really care. Life is too short.