FLC OF MLG IN DSCNT ALLOWED AUTOPLT TO LEVEL AND START CLB BEFORE CORRECTING.

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WE WERE GIVEN A DSCNT TO 17000 FT. AFTER THE DSCNT WAS INITIATED AND WE WERE PASSING THROUGH APPROX FL190; WE WERE GIVEN 13000; WHICH I DIALED IN. BOTH THE CAPT AND I WERE LOOKING IN OUR FLT BAGS TO SET UP FOR OUR ARR AT OUR DEST. SUDDENLY ATC CAME BACK AND ASKED US OUR ALT. WE BOTH LOOKED UP AND DISCOVERED THAT THE AUTOPLT HAD CHANGED FROM A DSCNT MODE TO A CLB AND WAS CLBING THROUGH FL185. THE AUTOPLT WAS DISCONNECTED AND THE DSCNT CONTINUED. DURING THIS TIME; THE AUTOPLT WAS OPERATING OFF OF THE VNAV PORTION OF THE FMS (PROFILE) AND WE WERE ON AN ASSIGNED HDG. WHEN THE AUTOPLT WAS REENGAGED; IT TRIED TO CLB AGAIN. WE SWITCHED THE AUTOPLT TO 'LEVEL CHANGE'; WHICH TOOK THE FMS OUT OF THE LOOP AND EVERYTHING RETURNED TO NORMAL. WE NEVER DISCOVERED WHY THE FMS DID THIS; SO WE WROTE IT UP UPON LNDG. THE TRANSITION FROM DSCNT TO CLB WAS SO SMOOTH AND GRADUAL THAT NEITHER ONE OF US FELT IT. SOMEONE SHOULD HAVE BEEN MONITORING THE AUTOPLTS PERFORMANCE INSTEAD OF BOTH OF US HAVING OUR HEADS IN OUR BAGS.