I don't understand, could you explain it?
When somebody files a bug on github there's a template to fill with the required information to build a correct bug report.
Starting the bug discussion here helps to gather that information and involves more people to reproduce the bug. It is a triage stage. It also avoids filing duplicates, and invalid bugs. It keeps the bug tracker clean.
So where to file a bug? The rule is simple: here! But if all required info are known, you can also file it directly on github. However posting here helps to inform other people.
Same rule for redmine, the old tracker for bugs affecting ns6.
I'd say, same rule for problems affecting the infra, too: mirrors, packages.nethserver.org, www...