Is there a documented case where the Aliens explain their "mindwipe" after a telling people an important Message?

I have been reading about alien abductions since i was a child, and there are many cases where people experience lost time, undergo hypnosis, and remember being abducted, and told an important message. If we assume that not every such case was created through false memories etc. then why would the Aliens give someone an important message, but erase, or at leastobscure their memory, so that they could not remember the abduction AND the important Message? So far i have not read about such a cas...