In my current game, I was going to the animated forest with banshee, as was customary in my family of elven wizards for the past 7 generations.
On my way there, I stumbled upon a lesser undead vault that wasn't entirely pacified earlier.
As such, some Gs and Zs had to be killed to make way and one of them dropped a ring of invisibility. Here's what happened next:

The banshee seems to scream! The banshee picks up the ring of invisibility.
The banshee seems to scream! The banshee equips the ring of invisibility.

Now, upon closer inspection:
This is banshe. She is hostile. She looks somewhat experienced. [M]ore.

So she did not become invisible. Is this alright? Does she have some sort of invisibility resistance or simply +invis gained via an item does not apply to monsters/npcs?
I was unable to see invisible other than through AMW, but after removing it, nothing changed, banshee remained visible.