Modern autopilots leave you in a trimmed state as they disconnect. I'd be surprised if this wasn't the case with the Q400
Alex, prepare to be surprised.
I've already mentioned earlier in this thread that the Q400 a/p is prone to leaving the a/c out of trim to varying degrees. Whilst it doesn't happen to the point where the a/c is in any way uncontrollable, it is nonetheless a pain in the backside. Disconnecting on approach often leads to a 'jerk' of the controls, usually forward, causing the pilot to have to retrim the a/c. The out of trim condition is made even worse by lazy pilots engaging the a/p in the first instance without correctly trimming the a/c beforehand.
The 400 is a perfectly adequate and capable a/c in many respects, but it does have its foibles and annoyances. The a/p is one of them.