For 15 years in software development, I’ve seen that many customers confuse features and bugs.
A bug is an error in a computer program or system that is considered to be a wrong software behavior in a known case. Known case means a user-scenario the software was designed for: login button, pop-up login form, password reminder form etc. If the form doesn’t open, it is a bug.
What is a feature?
Let’s say, one user posts a message to our ticket system: “I can’t restore my password. It doesn’t open on my iPhone”. Of course, no one makes a website without mobile-friendly version. In that case “Restore password” on iPhone is a feature due it was not on the SRS (software requirements specification).
One might say that it is obvious. Is obvious to you, Einstein or Elon Musk?
The undocumented requirement is a feature rather than a bug.