I posted a bounty on my question about work rotas, the bounty expired with no answers and so was not awarded. I'd have expected that I get the 50 rep back but I don't seem to have... is this correct behaviour?
A bounty is not a guarantee. It's less risky in the new system, but it never has, nor ever will be, a guarantee for the answers you need. It's payment for a bid for an answer, not payment for actual answers themselves.
Remember that although you didn't get an actual answer, you did get 7 days on the Featured tab. If you got refunded, then you could keep your question on the Featured tab forever, which really reduces the benefit that placing a bounty has in the first place.
Basically, consider this like a parking meter - the reputation you spent is good enough for only 7 days. If you need more time, you need to spend more reputation. Of course, it's a parking meter where you can pay however much you want and get the same amount of time, but the extra payment is for the possibility of better quality attention during that same period.
So yes, everything working as intended.