Portfolio posts appearing under wrong page slug in the url

The site I’m working on has two pages that can pull in ‘Portfolio’ posts: Work and Blog. You define which page you want to assign the post to using a dropdown.

Work is working perfectly. If I go to work page (siteurl.com/work), all of the correct portfolio items appear. If I click on an item, it takes me to siteurl.com/work/work-title and the page loads correctly.

However, on the Blog page, which is set up identically, after clicking on an item, the siteurl.com/blog/blog-title page gets a 404.

Here’s the confusing part… If I change the url to ‘siteurl.com/work/blog-title’ the page loads correctly.

What would be the cause of the portfolio post being assigned to the right page, but the wrong permalink?

If I set the site to Default Permalinks, everything works as it should.

Thanks in advance for the help!