To elaborate on this: the reason people ask you to do that is not because “people think it might magically fix the issue”, it’s for triaging.
GDevelop’s code is pretty strictly peer reviewed and tested before being accepted, and eventual bugs that slip through get quickly found and patched. That is especially true the more widely used the feature is.
When you are claiming that one of the most core, widely used, peer reviewed and tested features of GDevelop has a bug without having been changed in months and no one else reporting anything of the sort, there’s a practically 100% chance you’ve just written bad code, and that everything is working as intended.
Asking you to replicate the issue in an empty project allows to ensure you can show an actual issue with the feature, not just an issue with your game logic. If you cannot replicate your issue outside of the context of your game logic, the issue is not with GDevelop: it’s with your events.
Since your claim sounds heavily like you have made a mistake, are not owning up to it and requesting the GDevelop team to take a lot of time out of their charged agenda to care of your debugging for you for free, it is expected you’d provide in exchange the minimum effort to provide proof that you actually have an engine bug, and a way for the team to see exactly what systems are relevant to quickly find the source of the issue.
That is a false analogy, and ignorant of how software works.
What you had with the Flexbox extension was a memory leak, which is inherently easy to prove, and best requires the whole project, as the source could be any component of the project, to run the game in the debugger which will help find the object that is causing the leak at runtime.
In that case, where there is definitely something wrong with GDevelop itself (GDevelop is supposed to handle memory so that switching scenes frees all memory attached to the previous scene, and clearly did not), and the entire project is useful for debugging that category of bugs.
However, what you are having here is a behavioral issue: The game logic is not behaving as you intended. In that case, there’s no proof that GDevelop is doing something inherently wrong: Your game’s issues are most likely due to an error on your side, where you wrote logic going against what you intended, in which case GDevelop can’t do anything and isn’t responsible.
Finding the source of the bug would require at least reading your events code and understanding your game’s logic thoroughly to understand what is going on with the game. This is a heavy mental charge and a process that would take hours with most games being relatively big projects.
That is an unreasonable amount of time, work and effort to charge the devs with. Not to mention it is most likely useless for GDevelop itself and only will help you debug your game for you.
That’s why, for these kinds of bugs, you are expected to do the basic debugging yourself: find out what other systems triggers the unintended behavior, and making an isolated project allows you to isolate and find the system introduces the issue. If that turned out to be an error on your side, the most likely scenario, good for you! You found the issue for your problem and can now fix it yourself.
If there was indeed an issue with GDevelop, e.g. you could show in an isolated project that jumpthroughs always fail when the defaults controls are off or when a platform has a specific type of collision, where the GDevelop code is definitely faulty, then you can show the devs that you are not just asking them to spend hours doing your debugging for you but have, and give them a project where they can actually “look at the project and immediately work things out”.
tl;dr You did not have an actual bug, you were just complaining you can’t get your game to work as you’d like, which is an issue for you to fix. People are asking you to replicate in an empty project to get you to pinpoint the source of the issue.
If you pinpoint your game’s issues to an actual bug in GDevelop, then it’s a GDevelop problem. Otherwise, it’s an issue with your own code and it’s unreasonable to ask the GDevelop team to provide unpaid hours of support debugging your game in your stead to find what you did wrong for you.