They've explained that it a known issue they have no way of fixing without allowing all off-line characters to get credit, enabling people to do the task on 15 characters and get credit on all of them every tick.
Only characters that are active in the game world are eligible for the task tick, a character is inactive when they are logged out (duh) but also when changing regions, specifically until your client sends an update again from the new region at the very end of the loading screen. There isn't really an way to easily distinguish between zoning and logging out that would not allow you to get the credit on 2 chars if you time it properly. If we would say you have a grace period of 60 seconds from the time you zone, if you time it properly you could just kill the daoc client while zoning, log in another char and turn in one item and you would get it on both.
While it would be possible to fix this, with the switch to NF and hence practically no region changing, this should mostly be a non issue making the required change not really worth it.