We get a lot of reports where people are confused why they suddenly seem to run out of HP out of no where. This is usually because a Party member checked in at some point during the day and caused a boss to damage everyone participating in the quest. The server then syncs this on any action (usually a positive task score) and the player seems to get hurt for that action.
Showing a toast that is specific to the boss damage being applied would be a lot more helpful in informing players what's happening to their avatar.