Apps still think my phone is rooted after I installed stock firmware. How to fix?

I need to run some crucial apps that detect root (boo!). My phone was indeed rooted, so I installed stock firmware (Oreo). But the apps still claim my phone is rooted!!!

If I install SuperSU it says my phone is NOT rooted.

So I am in the worst of both worlds. How can I clean up my phone, so that apps no longer incorrectly claim that my phone is rooted?

Note I do still have an unlocked bootloader and TWRP. Could that be doing it?

Nexus 6p
Oreo 8.0.0

Thanks,
Bob