Thank you for your post.
iOS gives an app a few seconds to respond to an enter/exit event. Immediately turning the beacon off followed by an immediate scan will return a value, making it seem the calculation is returning a value when the beacon is off. This behavior occurs in FileMaker Go 15.0.1, 15.0.2 and 15.0.3.
How much time after the beacon is turned off before you perform a scan for beacons?
Testing would like to get a copy of your solution so they can investigate further. I have sent you a private message with instructions where to send your file.