Because the hunt group is in night service.
I always create a separate hunt group called Paging, say 320, collective call waiting, voicemail off, No answer timer 999, no overflow, fallback to main (in case they're trying to page in an emergency) and then add paging members to it.
Then have a separate set up from your main group and night group.
Button on handsets
Feature: Dial Paging
Dial 320 Paging
Done.
If you can't add members to this new group you are either experiencing a 'feature' or PEBCAK. Delete group. Merge and try again.