Did the cotton completely fill the cotton slot? If you had to adjust it to cover the liquid openings, it might have been too little. I never thin it out or make it less dense, I also build 2.5mm coils and that amount of cotton nicely fills the slot (but it’s not too dense either).
How high is your coil positioned? By default, the top of the air intake and the coil legs are close (if you look from the side), I always raise it a bit. Not this one, but I’ve experienced it with other RTAs where it leaked because the juice wicked down from the coil, they were so close to each other that it didn’t cause a short circuit yet but they were almost touching (I forgot to check).