Developer denies the bug I found and says it's a feature
Please answer the question: I have noticed the behavior of the system, which I consider an error. The documentation doesn't say anything about this. The developer claims that this is a feature. The project manager says that this is not important and I can do it on your own. What should I do?
If I think that some functionality is not working correctly, regardless of the developer's opinion, I write out a bug and ask the project manager to clarify this functionality. Further, the developer write a comment that this is not a bug, but a feature. And if later it turned out that this is a bug, and this bug went to the customer, then I just show the bug to developer and so that was not my fault. I advise you to do the same.