Bump. We're experiencing the same exact issue. I've submitted a request to technical support to get a downgraded firmware version. Once we go to v.7.20, iOS 11 devices cannot print after their first request. All subsequent requests do not work. It works again if you turn bluetooth off and back on, then you get your one working print job again. Nothing thereafter.
This is not an issue on iOS 10 or with printers that have not upgraded firmware versions past v.7.19. Also, non-issue for Android.

This is an interesting issue. Would you mind of sharing your app with us to further investigate this issue? You can put your app on Apple's Testflight and invite isv_nala_support@zebra.com as tester.