The app could detect me even after factory reset. How?

Maybe that APP have collected your hardware information and saved it in its remote server.

Hi,

as long as the app is using Google's Backup functions, it can simply detect you by your Google account. I assume you used the same account after the reset?

— modified on Sep 24, 2018, 9:57:31 AM

Your on line accounts are not stored in your phone and those accounts are still identifying by your device I'D number!
Even if you change the accounts!
Try this ; delete all of your on line accounts completely!
Then do a factory reset.
But honestly, I don't think that will achieve the results you are looking for either!

— modified on Sep 25, 2018, 3:44:00 AM

if you log into the same account, then it will automatically upload your old data. It's normal. But if not, maybe the app have already collected your data by asking for several application permissions which you have granted in the first place.

At least it can collect your phone id information. But I am not clear about how to spoof your hardware information.

Once again.
Your being identified by your device I D number!