I'd guess "or maybe was not aware about this duty" is the case. It's not clear for the casual user that this expectation exists.
I think your proposed approach is fine. I would go a step further and send the reminder to the submitter after 2 weeks and then automatically close any "resolved" issues after a month if the submitter has not given any update. The option for the submitter to re-open still remains even after it was closed. This is very common among commercial software vendors, usually much faster than this (reminders after a day, auto-close after 3, but without the option to re-open. Yay commercial software.)
Having the resolver close the tickets is another path, but that adds an extra burden to the resolver, or feels very dismissive to the submitter if they feel the resolver was over-eager in closing the ticket.
