Lightroom mask fails to detect subject or background

Issue: Lightroom fails to detect subject/background

Lightroom CC version 6.3.1

OS: Windows 11

Lightroom has been failing to detect subject/background. Many months ago, this feature was working properly.

Back in October 2022, I succesfully created a "subject mask" on the photo below. Without making any changes to the photo, I attempted to create the same mask in May 2023. As you can see, it is not working as intended.

Every photo that I try to create a subject/background mask on ends up with similar results. Here's another example of what's happening:

Please help--thank you!