here's my thought. Notice how when a unit is closed, it seems to keep any temporary buffs until it's revived the next turn, or simply goes to the cemetary. For example, this could cause a closed Guardian that had been targeted by Miracle Fruit (lvl +3) the previous turn would be uneffected by Instant Revival (can only target lvl 4 and lower).
Maybe a closed unit keeps all effects it had active when it dies, so the Song of a Hero effect was still active, even though it should've ended.
[shameless link] I can still claim I'm a free player cause I haven't spent any of my
money on the game, but instead I used Prizerebel to get my EX packs. Gran Train = viewtopic.php?f=13&t=11761