Their attachment's movement was truncated to whole units before the
test, so it took two units of movement in one update, not one, to move
the vertices along with it. Visual only.
(cherry picked from commit 99c2db2d4da3d6d66b6ec28d459c756aeab55a64)
(cherry picked from commit eb0667bcc81ed1d970422c7856162e8aadff6fd7)