Can you replay scenarios in Frosthaven?
Only in casual mode, and casual mode gives you nothing: no XP, no loot, no treasure, no checkmarks, no personal quest progress.
In the campaign each scenario is played once. It can be replayed later as part of the campaign only if the party still meets the scenario's requirements and it was not already completed.
Casual mode is the free-play option: any unlocked scenario, regardless of requirements, as often as you like. In exchange every campaign effect is switched off — no events, no time passes, and the party gains no experience, loot, treasures, checkmarks, personal quest progress, rewards or unlockables.
That last part is the one that surprises people. If you replay a scenario because you enjoyed it, you are not levelling that evening. Frosthaven still counts the outcome as having no effect on the campaign.
“When a scenario is played in casual mode, all campaign effects are ignored. No event is resolved, time does not pass, and the party does not gain any experience, loot, treasures, checkmarks, personal quest progress, rewards, unlockables, etc.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.