I thought you had to actually use a turn for it to count.
Like, if you use a skill to damage the boss and the raid ends it would refund you BP and give you nothing. (At least that's how it is for normal raids.)
I've joined a Ragnarok, not done any damage, and been refunded my BP. It's still a net loss because those BP restoration items you used won't be refunded, and now you have full BP and nothing useful to spend it on.