I think there might be some confusion here between two different abilities/skills, so let me attempt to clarify, and if you still believe it's not working this way, I'll have someone from the Combat team take a look.
- The Tracer Missile ability stacks a debuff on your target. Without the skill "Light 'Em Up", it takes 5 shots to build the stack to 5. With the skill, it takes 3 shots (as each shot now grants 2 stacks). I just logged in to check, and it still does work this way.
- The skill "Tracer Lock" grants a buff on yourself, increasing the damage your next Rail Shot will do. This is not affected by "Light 'Em Up," and I don't believe it ever has been.
Can you please look at both your buff and your target's debuff and confirm this is working the way it should be? The debuff should stack twice as fast as the buff, neither of which have been changed. If you guys have more questions, let me know and I'll get someone to take a look.
While Light 'Em Up has never effected Tracer Lock, with the changes that went in during 1.2 (having us fire off a barrage unload basically every 5 seconds and lowering the damage of tracer) it should...getting 5 stacks of Tracer Lock is impossible before the skill is ready to fire again...at most we get 3 and the skill may as well be on a 25s cool down.