Working as “intended” per here:
I still maintain that whether or not the actual mechanic in which the damage is calculated is intended, it is a bit frustrating that it is considered “not a bug” to show a number in the boost ratio preview that is always incorrect for this specific spell. Again, simplest fix here would be to change the text (boosted by all other allies mana) and also change the boost ratio condition (from allies/all/mana to allies/other/mana, which is hopefully already something that can already be done).
Similar boost ratio calcs that take “fragile” buffs into account (those that go away when you cast like enchant or submerge, like on Penlong and Undine) could get a similar treatment, because this is an issue for them if they have their respective buffs before casting.