Issue tracker: weird ordering of milestones

On this particular project, I've created milestones as such: 2.5.x, 2.6.x, 3.x.

In the admin interface, the milestones appear listed in this (natural) order, as expected. However, in the tracker, when sorting the issues by Milestone, the observed order is 2.5.x, 3.x, 2.6.x