When you’ve spent an entire day trying to figure out what the issue is, it’s no longer a bug.
When it’s going to take longer to fix it, than it did to build it, it’s no longer a bug.
When you need to pull in the entire team to diagnose and troubleshoot the issue, it’s no longer a bug.
When you are trying things that you aren’t sure are going to work, but hope that they do, it’s no longer a bug.
There are many other reasons and scenarios here, the point that matters is that you define when a bug is no longer bug and set your team up for this shift.