The stealth from trapper runes is not functioning quite right. Normally when a thief gains stealth, the auto attack stops. However, gaining stealth through traps with these runes does not end the auto attack. If you have been attacking and the target is in view and in range then the auto attack fires and the sneak attack is lost. I have not tested this on a ranger.
A couple of ways to work around it are to sheath weapons before laying the trap or be facing away from the target. These are not ideal because in the heat of combat the auto attack will often end stealth if one of these moves is poorly timed.
The best solution would be to end auto attacks the same as with other stealth methods.
Thief main plus a few alts – Sanctum of Rall