First observed on a B737-300, the one with the new MCP. In the same time frame, it was observed on a B767. It did lead to a loss of separation incident with a QF 747 and a AN B737 climbing out of Adelaide.
The advice then was from Boeing to keep checking the MCP selected ALT. They had no idea why it was happening. The theory was electrostatic discharge from pilots fingers because we sat on proper sheepskin seatcovers, but wore nylon wool mix trousers with rubber soled shoes. Seemes plausible.
And it was 1986, so Boeing has known about it for a long time. It was a new B777 Manager at Boeing, with a great deal of experience on the B737, who caused the Bulletin to be written.
Oh, and I did see the MCP ALT change without pilot input while working for AN, but never on the B777.
The important phrase is "when MCP selections are complete, select 1000." Use common sense and it is no big deal.