I tried a few tools to check if the pdb and the dll actually matched, and using chkmatch I could see that the GUIDs in the dll being run and the pdb in the obj folder didn’t match.
So it turns out that although the dll and pdb in the project’s obj folder are a match, the dll that was actually getting copied to the application’s destination folder by a post-build event was the old dll from the previous build.
The post-build event was running before that particular project had built, or at least finished building, and was copying in the existing dll from the bin which was subsequently overwritten by the continuing build.
I have resolved the problem by editing the project dependencies for the solution and ensuring that the project with the post-build event is dependent on the project that wasn’t loading, and now the pdb loads during debug.