… or is it an exception to how stickiness works?
Never encountered this situation until I was fooling around modeling a very simple railing segment.
Stickiness works as expected with just one rail in the equation. When I add the second rail, and the initial baluster is connected to both rails, the copies don’t stick to either one.
A way I found to make the baluster copies stick to the rails is to first group the two rails, then model and group the first baluster with the baluster copies. Finally, apply the outer shell command to the two groups.
It also works if the rails are grouped, and after the balusters are in place the group is exploded.
The reason I want the stickiness (rather than separate groups for rails and balusters) is because in some instances I find it faster to select geometry and just move (stretch) it for modifying height or length