I asked a question about some behavior of Vim I could not understand. I received comments saying that this behavior was probably because of a bug. What should I do with this question now? Should I leave it open? Should I answer it myself, saying in the answer that this behavior is because of a bug?
First I'd search whether the problem has already been reported. BTW: read the guideline for contributing that already explains that and much more.
Then, if I'm definitively sure this is a bug, I'd directly open an issue on vim repository on github. In doubt, I'd first ask on vim-dev mailing list.
In other words, I'd go to official places. Even if Christian is around, the dedicated tools and channels need to be used to report problems.
In the case of bugs detected in vim scripts shipped with vim, the correct procedure is to contact first the script maintainer(s). We can't expect Vim core developers to know how a foobar plugin/syntax script is expected to behave. They may accept a patch that doesn't make sense, so instead they trust people in charge of these scripts.
Eventually I'd report on Vi.SE/SO question that the issue has been officially reported, and that the evolution of its status can be followed at link-given.