Trouble with numeric permalinks after 4.3.1 update

Since updating to 4.3.1 we are having an issue with permalinks. Our usual practice has been to have a unique article number serve as the postname, and that would be used in the permalink – for instance:

http://our-site.com/1408558

As of the update to 4.3.1, WP automatically appends “-2” to the slug, mimicking behaviour as if a similar post already existed, which it doesn’t.

http://our-site.com/1408558-2

While that obviously works fine, it disrupts our normal means of structure and organization for the site.

Here’s what I’ve done so far to investigate:

• Deactivated all plugins – same behavior exists.
• Changed to 2015 default theme – same behavior exists
• Went back to a site running 4.3.0 – didn’t encounter this problem
• Refreshed permalink structure – same behavior exists.
• Added a non-numeric character in the permalink – problem goes away
• Changed permalink structure to /testing/%postname%/ – problem goes away

I’d rather not have to add a non-numberic character, and we can’t roll back to 4.3 – so I’m really hoping someone else can help me figure out what changed in numeric permalinks going to the most recent version.