Hi. This one has pestered me for a while, and since one of my other recent "I thought that was obvious" type of suggestions got traction, I figured I should throw this in the mix.
Assuming you are running Netscan on a Windows PC (Mine is Win 10, build 14393)
Run a scan and get results, at least 5, but preferably many results because we like results, and it's easier to see the bug.
Let's say you get 10 results and you want to select various groups and give them orders to carry out your devious plans.
You click-drag #2 through #5 and you get 4 items (2, 3, 4, 5) highlighted. Orders delivered, minions dispatched.
Next you Click-Drag #10 through #6, then CTRL+Click #1. You get all 6 items (1, 6, 7, 8, 9, 10). Now your devious mind becomes distracted by cute kitten videoz on the You-Toobz and you lose your selection.
Returning to the task at hand, You CTRL+Click #1, then CTRL+Click-Drag #6 through #10. You should get 6 items but you don't. #6 failed to join the group! We'll see about this little rebellion later...
If you CTRL+Click individual items, then CTRL+Click-drag for a group, the first member of the group is always dropped.
Indeed this looks like a bug, I was able to reproduce this behaviour. The problem though is that it's somewhere deep down in the tree control's code, so fixing it is rather challenging.
I will let the tree control's developers know, but granted it's open source they may be reluctant to spend time on this. As a workaround you can simply click the de-selected item again and it will be included in the group again.