Even though you write bug-free code and test everything on your device, there are certain things you cannot test. One of them is the sheer number of mobile devices out there with all kinds of different screen sizes and various operating systems. Some of them over 5 years old.
So while you think every device works perfectly, in the real world it simply won’t. And there’s no way you can prepare for this, so the best thing you can do is to fix issues as soon as they occur. Having access to the development logs of the affected devices and inspecting what actually causes the problem will save you days if not weeks of speculation, trial, and error.