UPDATE: I finally managed to determine the glitch with my setup. There is a compatibility issue with the combination of plugins Facebook Thumb Fixer (1.5.1), W4 Post Lists (2.0.7), and WordPress 4.3.1 (possibly starting with 4.3) that create the permalink issue with my client’s site.
Through a laborious process of elimination (including with all other plugins deactivated & with a default WP theme), the following conditions create the permalinks glitch:
– Using WordPress 4.3.1 (possibly starting with 4.3).
– Facebook Thumb Fixer plugin activated.
– W4 Post Lists plugin activated.
– Have/create a few posts.
– Create a custom post list with the W4 Post Lists plugin.
– Have/create a page with pretty permalink.
– Add the shortcode for the custom post list (example: [postlist id=123]) and Update page.
– The page permalink is automatically changed to the permalink of one of the posts instead.
A previous version of the site on WP 4.2.5 with the same versions of those two plugins, did not create the glitch. So something has changed from the earlier WP version that is incompatible with those plugins in this situation.
If there is no custom post list shortcode on the page, then there is no permalink issue. But since being able to create custom post lists with W4 Post Lists is essential for my client’s site, my only choice is to disable Facebook Thumb Fixer. This may not be the answer for you guys, but perhaps this can focus your investigation and point you in the right direction.