So, I’m going to go ahead and just acknowledge upfront that upgrading and rolling back that many versions at once was probably, erhem, ill-advised. Alas …
After testing in our test environment, everything seems to be working like normal, with the exception of some book’s ToC links are redirecting to the root site. Specifically “parts” are redirecting. Posts don’t seem to have this issue. I have attempted to reset the permalink structure on one of the affected books from this url, to no avail:
https://mysite.com/<BOOK_NAME>/wp-admin/options-permalink.php
What does work is, from the admin area, navigate to the editor on an affected “part”, clicking save, then visiting that part page. This isn’t ideal for us because we have thousands of books and it’s possible that many, some, few, or all are affected (we’re still trying to figure that out).
I’ve also gone spelunking in the DB and found that many (maybe all) wp_123_options tables seem to have the value /%year%/%monthnum%/%postname% or something close to that on the permalink_structure, which is weird to me because I’d have expected the value to just be /%postname%/.
Finally, and this may be unrelated, but I saw this thread and thought maybe there’s something in PB code that changed? Permalink in part CPT
Anyway, I’m still digging into this issue but wanted to reach out in case others had thoughts around this. Thank you very much in advance.
Bryan