Root TWRP keeps getting overwritten


I'm trying to install TWRP, but it keeps getting overwritten. After installing, I can initially boot into TWRP, but after doing a system reboot, it goes back to stock recovery. What am I doing wrong?

Here's the steps I take:
  1. Open Odin, check AP and load twrp image (3.0.0)
  2. Connect phone and wait for Added!
  3. Uncheck Auto Reboot
  4. Click Start and wait to finish.
I then have to take out the battery to turn the phone off (I had to do it this way because leaving Auto Reboot checked would cause the same issue as below). I then boot directly into recovery and TWRP loads as expected. Everything looks fine, and I can do a recovery reboot back to TWRP. However, if I do a system reboot and let Android load, then shutdown and boot into recovery again, I'm back to stock.

Sprint SM-N910P
Android 5.1.1