What broke
A gitignored file, or any untracked file under a common configuration, is destroyed by a git worktree remove that exits 0 with no --force — reproduced on two Git versions. An earlier revision of the tool's own documentation claimed a backstop existed, and that claim is what made the hole invisible to the tool's own review.
The rule it paid for
Condition 2 asks for untracked and ignored explicitly; the only exception is a committed manifest a person wrote down.