Formation members occasionally get an unreachable offset position. They will then try going there repeatedly, stopping only when the formation controller has stopped moving and they have (following rP23806) exhausted the regular fail-and-try-pattern.
However, with formations, this can lead to many units running short paths in short succession, which is laggy. Since formations units remain in formation, it's OK to stop as soon as it looks like the unit is failing to reach its offset (iff the controller has stopped, this is handled by unitMotion not reporting messages then, see rP22464/rP23496).
So, in FORMATIONMEMBER.WALKING, it's better to FinishOrder on any MovementUpdate. This mirrors pre-rP23806 behaviour, since the code was buggy before.
This is arguably a hack, but it's now documented.
Reported by Freagarach on rP23806.
Follows rP22464 & rP23496.
Related to #5624 in that the max-short-path range is the source of the lag.