Currently it is possible to add Issues to Systems in Assets, but not add systems as reference within Issues on WEB.
It is possible in the mobile APP and you see the reference.
On WEB you don't even see the system reference in Issues. I was thinking it was a bug, but Autodesk support responded that it is designed this way and it might be on a future Roadmap...
Would also be nice if the system reference also could reference to the assets in the system as a choice.