The 15-1994 can't learn a key being held down (it doesn't learn how long you held the key). The 15-1994 does try to learn the repeat pattern of the signal. If you release the original key too soon the 15-1994 may learn the initial signal and fail to learn the repeat pattern and give exactly the symptoms you described (but for a different reason).

"NEC" is our name for a family of related infrared protocols. The 15-1994 has a general purpose routine that sends signals of that family. One input to that routine is a collection of bit flags that determine various characteristics that differ from one NEC protocol to another. If any of those are wrong, you get a basically working setup code with subtle malfunctions such as you described. For Samsung TV's, one of those bits is wrong in TV/0060 (I think TV/0060 is perfect for some other brand TV, just wrong for the Samsung TV's for which this issue has been discussed).

With JP1 you can create a new setup code changing any detail you want. For more information, join the Yahoo JP1 group and ask about fixing the repeat style of TV/0060.