This has happened to me several times lately, once with Alba and Cosmos in the party I was in. This basically means it's already known, but I wanted to officially report it and raise awareness.
Occasionally, healing magic will seem to only heal 1/10 the proper amount in battles. When this happened, according to Alba, the field has somehow been flagged as a PVP Arena. We don't know why or how but it happens.
So to all you white magic users and drain spammers, bring potions, just in case, because you never know.