types.MoveWindowOptions.session

Omitted means this window's session, not tmux's current one — tmux reads a destination of :3 as index 3 of whichever session it happens to consider current, which is rarely the one a caller holding this window means.

A Session handle is checked against this window's server before the command runs. A string is not: a tmux id means something else on every other daemon, and only a handle carries which one it came from.