Cannot add new subdirectory sites since 6.4.3

Since updated to 6.4.3 when I try to add a new site to my Multisite I’m running into a problem I have never seen before.

New sites show up in the Sites list but are not actually being created as sites. When I try to edit the site I see a URL that starts with :// instead of https:// and in the settings I’m missing all of the fields I would expect to see for a site (siteurl, blogname, etc.) and the home field is set to my root domain rather than the expected domain.ca/site format. The site creation email sent to me when I create a site shows an IP address but no domain or site name and if I try to manually navigate to the URL of the new site I get “Error establishing a database connection”

Sites created before the update to 6.4.3 are working as expected including being able to create new pages and other content. I’ve disabled all the network activated plugins on my development site and reinstalled the update in case one of the plugins was the culprit but I’m still seeing this behaviour.

I’m not sure where to start trying to unravel this problem. Any advice on where to start would be a huge help.

The page I need help with: [log in to see the link]