I believe they're told to keep the flag down until they're sure the play hasn't led to an immediate goalscoring opportunity.
The idea is to avoid preventing a legitimate goalscoring chance by stopping the play for an offside call which turns out to be incorrect. They let it play out and if a goal is scored they can then review it with VAR to ensure the correct decision is made regarding offside or not.
If the player misses or the play otherwise breaks down, the lino can raise their flag at that point if they thought it WAS originally offside as there's no longer the risk of denying a goal.
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