Server-side commands (RegisterCommand on the server) can not be used with RegisterKeyMapping, they simply do not execute.
Cause is already known from discussions, this post is purely to prevent the topic from being lost to chat archive hell.
Repro resource: https://matrix-client.matrix.org/_matrix/media/r0/download/matrix.org/nlqvBeQJKGsvBWulHbPjudvd
Ref
Matrix reference link: You're invited to talk on Matrix