It depends on your DM.
If it were my game, I'd say "Fear-immune" just means "immune to infliction" and not "immune to effects".
This means that when they leave the Malice's radius, and regain their "immunity to infliction", they would still suffer from the panicked condition's effects until they wore off; but after leaving, then nobody could inflict another instance of panicked on that creature until it lost it's "immunity to infliction" again.
Once the effects wear off though; upon re-entering the Malice again, they'd be granted another Will Save to negate it's effects, at which point (if passed) it no longer concerns that creature.
However: If your DM interprets "Fear-immune" as "Immune to effects", then upon leaving the Malice, the condition could be removed entirely (unless the DM says otherwise).
If you're going strictly by the book: It seems as though the condition would remain and would run its course, though you'd never experience any negative effects from it while immune.
Therefore: Should you become panicked while in the Malice, you'd "recover" from it the moment you left it; but you would still have the condition. Should you re-enter the field before the condition wears off, you would experience its effects once more. In that scenario, though, it could be argued that you're prepared; and you may be allowed to make a Will Save to resist it long enough to act normally for that round.