In a custom game mode I was testing out a lifesteal build with Lucian and i purchased a Bloodthirster, Blade Of The Ruined King, and multiple Vamp Scepters, when I added Death's Dance it lowered the total lifesteal percentage, when I undid the buy the percentage went back up
Death's Dance doesn't have actual lifesteal (it has physical damage vamp, which works on your abilities; the unique passive), but because its components do, your lifesteal diminishes when you complete it.
That is to say, if you have a Vampiric Scepter, it's giving you 10% lifesteal; when you complete Dance, it gives you 15% vamp, but that's not counted in your lifesteal, and since you lost the 10% proper lifesteal, your total goes down.
