I am not positive that this is a bug, but it's probably sufficiently exploitable to be considered for change.

Essentially, the bug works like this:

If an invisible PC encounters a monster that cannot see invisible, the PC may cast bolt spells (fire bolt, magic missile), or use equivalent wands to damage the creature, and the creature will never acknowledge the existence of the PC and attack. This also occurs if the PC uses mindcraft.

To my knowledge, all other kinds of attack (ball spells, missiles, melee, acid spit, etc.) trigger a hostile response from any monsters struck by the attack.