I cancelled an Avios booking about two months ago where the Avios and cash were refunded pretty much the next day but the voucher never arrived back in my account. I sent an e-mail to 'remind' them to re-deposit the voucher - that was five weeks ago, have heard nothing yet, and in those five weeks the voucher has expired! I'm fully expecting an unhelpful reply now stating that they can't refund the voucher because of its expiry (ignoring the fact that had they refunded it on time I would have had time to use it)

So my answer to OP's original question is: Sometimes they don't, and you have to chase it up.