One issue here may be that the closing date may not have been imported when the old tickets have been imported from trac to pagure, there is still something which bugs me a little though

I was wondering if the closed_at field was getting properly filled when importing closed ticket, but according to the code and the tests, it does (cf PR at https://pagure.io/pagure/pull-request/3402 which ensures the field gets filled)

In the mean time I recommend you use:

https://pagure.io/fesco/issues?status=Closed&order_key=last_updated&order=desc
and
https://pagure.io/fesco/issues?status=Closed&order_key=last_updated&order=asc

The new UI (merged in git) will offer ways to sort by the last_updated field even for closed tickets.

The "closed" column is empty for many tickets, so it looks like the either date was not imported or maybe the display code cannot process whatever timestamp there is?

The "closed" column is empty for many tickets, so it looks like the either date was not imported

I think it wasn't imported, potentially because the closed_at field wasn't present at the time of the import (it got added in June 2016).

Can we add a special case for those old tickets that looks at the last_updated field when closed_at is empty ?
Another approach would be to fix the entries in the DB directly by setting closed_date to last_updated if closed_date is empty.

Can we add a special case for those old tickets that looks at the last_updated field when closed_at is empty ?

I think doing this would be pretty complex.

Another approach would be to fix the entries in the DB directly by setting closed_date to last_updated if closed_date is empty.

This may be a simpler fix, especially considering this should only happen on old or imported projects.