Does the network profile have all the same control keys? Check Activities under OPS4.
And Control Key on that Activity has a check next to Cost?
New projects are working fine? Since it looks like it is a database inconsistency try changing the Control Key on that Activity to something else, Save, Cost, Change control key back, Save, Cost.
Or delete the activity and recreate with a different activity number. I have seen strange behavior when deleting activities then recreating the same activity, it was like the activity was never deleted.
Worst case, delete the activity, then create a new network header on that project and use it for the new activity.
One off problems are a bear to trouble shoot.