iPhone setting preventing 3rd party app logging in

So a 3rd party app (QuickBooks Self Employer) stopped logging in for a few weeks - it just wasn’t processing my login data, as if it was blocking the request before validating. This was on my iPhone 14.

i got a 15 a few weeks later, copied the data over etc and still the app will not login in. Interestingly,before I traded in my old phone, I wiped my 14, still signed in with my iCloud but didn’t copy any data, downloaded the app and it works as expected, showing it’s not the serial or IMIE it’s blocking

I’ve spoken to the app developer and there still looking into it, but to me the issue is something on my phone that has carried over with the transfer.

i have tried a new phone name, that didn’t fix it. What else carries over that I can check and try amend?

any advice or similar situations would be great to see what you did

thanks