Damage decreased after setting skill shard ¿?

The last days i've been leveling my Axis Bomber Boy (Assassin) in Toys Of Death (Madness, Battlefield) which is a good place since you can almost kill every opponent in one or two hits and get a decent amount of xp points. It is almost at level 100 and with 6 lvl 40 talismans.

What happened is that i was able to do 3200~ consistently (and rising as the lvl got higher) with perfect attack but after setting a skill shard and gaining "damage +5%" on air strike (the default attack) now i do 2700~ damage :(

Is there a known bug related to this? anyone has experienced something similar?

thanks