As far as I know, and I am not a novice on the B744,but stand to be corrected, A/C only will descent out of vnav alt if locked in approach mode, to descent out of vnav alt mcp alt has to be changed and button pushed. Then A/C will start descent only if there's no alt constraint in the FMS limiting the descent. In constraint case mcp pushbutton has to be pushed untill FMS alt is lower then mcp alt.
Automatic descents after alt. constraints will only happen if A/C is in vnav path, as done in non prec. approach.
Greetings.