I reset MCP ALT, was ready to descend and set my approach speed in the FMC and then the speed was set to 133 before the plane even began to descend, even after setting speed manually in the FMC VNAV kept trying to set speed to approach vref, alpha activated to stop from stalling but the vnav kept wanting to set speed at VREF at FL360. is this a known bug, I'm quite new to the 757, only about 50 flights logged, I know its difficult to descend sometimes but this has never happened before

757 captain III on P3D V4.5