The developers shouldn't be convinced if it's a bug or not. If you think something is not working as it should but you don't have any specification about how the flow should have been, you can do the following:
- add a comment to the task about the issue you found
- open a new task with the modification
But always talk to a project manager/business analyst/product owner who has the business knowledge and can decide about the workflow.
What I recommend is not to "force" bug reports. The aim is to have a good application which works as expected, not to annoy the developers. But always be aware:
- the product owner knows about the behavior
- your observation were written in an email or task tracking system
- your observation reached the desired people
Note: I recommend you to read the ISTQB Foundation Level book, even to take the exam. It's not a big deal at all, and at some point your experience will overtake that book with miles, but it's a great book to clarify the roles and tester approaches.