Just a random thought -- If you used the phone app to turn off IB, go to the website (using ONLY the Chrome browser) and make sure it's turned off there.
Probably not the cause of your issue, but there's a slim chance.
same issue here. Got an instant booking even though i had it set to off. Checked the app, it had been turned on ! ( not by me ) turned it off. Checked on the web version and definitely turned off, then got another instant booking !
Can't seem to get an answer to resolve the issue...