The assumption any dev should have is:

"The bug is yours"

That isn't a true statement, as there are exceptions. It's just that you are unlikely to be the exception.

Whether you're relying on a compiler, network, some other application, a library, an API... the bug is nearly always yours and you should more rigorously check your code without making any assumptions, until you fully understand the root cause of a bug.

If you actually have got to the root cause of a bug, you'll find in nearly all cases the problem was with your code.