tomorrow I'll try it without the invis. Because that seems to override skill if detected. As to proximity, unless I hit them, they shouldn't be able to detect me even if i jump on their heads. Which I didn't. Notice the red eye symbol came up to my right. A troll I didn't actually connect with.
I believe from my observations, that there's actually a dual detection issue going on there as well that might be causing the weirdness involved by detection.
1. If your move silent is low enough, it causes the sound echoes and actually might temporarily cause enemies to not consider you whilst you are still moving in their zones of detection due to that check for the echo. So enemies that unable to spot you or see Invisibility can actually do what you described and land on their heads without issue.
2. Conversely, if your move silent is high enough then because there is no echo to trigger them to search towards the echo and miss you, it instead allows the enemies to detect you as you have entered their detection zone which is automatic at a certain point (it's roughly a 120 degree cone in front of the enemies). Outside of this zone would be fine.
I believe in the video you provided you fell foul of 2. As your move silent is very likely at a level they can not normally detect and thus they only have the auto detection zone (when you landed in front of them between the two Trolls) and that's what set them off.
So there's a weird dichotomy where if you're Invis'ing, then having bad move silent skills might actually be a plus if enemies are scattered in a pattern ahead of you where you can not sneak by them normally (with enough distance between you in their detection zones).
J1NG