Computers aren't magic, much as they seem to be. In general, if you can reproduce a bug reliably, it is always possible to keep digging until you find the cause

Computers aren't magic, much as they seem to be. In general, if you can reproduce a bug reliably, it is always possible to keep digging until you find the cause. It may take more and more esoteric tools to do so, and you may need to start breaking apart the components you're working with, but at least ask yourself the question "what is preventing me from examining what's going on here" and try to find out how to look inside whatever opaque box you've run up against. Those boxes will start out being like "the function I just wrote", but as your understanding increases and you become familiar with how to look inside the least opaque boxes, the hurdles become more like "the JavaScript runtime", "the C compiler", "the syscalls made to the operating system" or even "the microcode emitted by the CPU" (though I've never needed to go to that level myself!).