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

[bobdrad]

I flashed a new Factory image (Oreo), but apps think my phone is still rooted. Using the Fx filesystem browser I checked /System/bin and find no trace of su or supersu, and I have no /System/xbin. If I do install SuperSU and try to run it, it says my phone is *not* rooted. OTA updates are working.

I have some crucial apps I cannot run, because they think my phone is rooted.

What could cause them to think this? What can I do to fix it?

Note I have an unlocked bootloader and TWRP recovery ROM. Could that be causing this?

Apps that I am having trouble with are AirWatch (which my company requires to access company e-mail) and the ADP Portal app.

I have Oreo 8.0.0 on a Nexus 6p.

Any guidance would be much appreciated.

Thanks,
Bob