To reproduce, start with a new empty window, set to continuous wrapping and turn line numbers on. Write a text with blanks over the wrap margin, so that you have a soft wrapping at a blank. There is only one, the first, line. This is correctly shown by the line numbering.
Make sure that the second displayed line doesn't end with a whitespace. Hit Ctrl+J, now you actually have two lines, but the line number for the second line is not shown.
This is already present in NEdit before the latest line number patch (by Bert) was committed.
5.6 [Under Development] HEAD
Sep 10, 2007
Built on: Linux, 386, GNU C
Built at: Jan 7 2008, 19:37:00
With Motif: 2.2.3 [@(#)Motif Version 2.2.3]
Running Motif: 2.2 [unknown]
Server: The X.Org Foundation 70199902
Visual: 24-bit TrueColor (ID 0x23, Default)
Locale: en_US
Sorry, I have no version prior than this to check when this was introduced. (Or was it always there?)
Logged In: YES
user_id=119143
Originator: NO
Confirmed with grandfather:
NEdit Version 5.1.1
March 17, 2000