Sorting columns that contain List of Users doesn't work

On design.xwiki.org there is a livetable that list a column for participants in the proposal process by using a List of users column.

Filtering by user id works, but sorting doesn't have any impact on the results.

I guess is complex to provide a sorting based on first or last name because we cannot say how to difference them (since we put them side by side and also depending on the locale the position might change), but we should at least have sorting by id.