Ghasts can see player after drinking extended potion of invisibility

I drank an extended potion of invisibility, then entered the nether. Several zombie pigmen which were previously aggro did not notice me, as intended, and the effect appeared on my inventory screen. However, a ghast that was several hundred blocks away noticed me and started shooting fireballs at me. Hitting F5 confirmed that I was invisible. I was wearing iron armor and carrying a sword/pickaxe at the time.