Ludens Echo + Lichbane - Bug?

Late game as Diana I had 613 AP, I bought Lichbane as the cherry ontop of my build, and.. for some reason it says the despite me having over 700 AP my Lichbane is only doing 138 damage. I sold Luden's and it jumped to 400 damage.. I'm extremely confused. Does buying both items together cancel a portion of LIchbanes damage or am I just going crazy?

Update: Tried in a custom and couldn't duplicate but did it in another real game on Akali and had the same thing happen. Not sure if it was a tooltip bug or if it was an actual damage bug.