Companions that use bow do tend to swap back to their default hidden bow, even when given a fully smithed, double enchanted daedric bow.. It's a bug. And theres no way to guarantee them using your given bow, but they will randomly equip it and swap back to default the next fight.
That's completely true. I've done that myself (minus double enchantment). And happens randomly. I even tried doing this with Aela the Huntress, since she is a marksman. She would equip the Daedric Bow in a fight, but 5 secs later, she would be back to her hunting bow that is NOT in her inventory, so you can't take that from her. It's just hidden, like her default armor. Arrows are equipped correctly though. Another [censored] game bug.