[DTV]

Yes, BBC News 24 produced its own overnight news programming to begin with - though BBC World did contribute World News segments at xx:15 during some hours. This was quickly deemed unnecessary and they merged 01:00-06:00 into the single BBC News overnight service from March 1998 (though with different back-half programming). Even during Europe Direct, which was the only simulcast programming at launch, World and News 24 opted-out for separate news bulletins.

Ironically of course, at that point overnights (when the channel was simulcast on BBC One) probably had the highest viewing figures of all.

That is true, though I expect they were still quite limited and the highest figures for News 24 content would probably have been the weekend breakfast simulcasts, particularly Weekend 24 on BBC Two. I also suspect there were other considerations in merging the operations - particularly the cost of running two studios overnight, with two editorial and production teams - the redundancy particularly obvious when you consider the World team were already producing content for both channels.