Hi D Mathew,
I've seen this kind of index stuck a couple of times. I didn't manage to get to the bottom of why it was happening, however one workaround helped me:
- stop IIS (or your app pool, or app service if you're on Azure)
- delete the folder with your index (in one of the cases here I saw files were still blocked by some process, so I only managed to release those after a reboot of the VM)
- strat IIS and your website again and run index rebuild