This does sound like a bug indeed. Let me explain you how it works, so we can track it down better:
When you press the button “water for x minutes” then the app will turn on the irrigation and at the same time create a rule which will turn it off at the desired time. So, while it’s watering, you should be able to open the “Magic” page and see a rule there which will turn it off. Perhaps something goes wrong when generting the rule. Can you check it and maybe send a screenshot of that rule so we can see what exactly it does?
Found the bug, fixed it. It’s building. The next beta version of the app (1.3.6) should have the fix, it will be released to stable with the 1.4 release.