Greetings , am converting from Bus to 777 and it just did this.
We had a constraint of FL230 by a certain waypoint. We were then cleared to FL 210 to expedite. So I used FLCH mode . At FL210 ( just before the constrained Waypoint I reselected VNAV to comply with the farther Constraints on the arrival and guess what , she initiated a climb towards FL230 ( with FL210 set in MCP). So a quick selection of FLCH saved the level bust. I guess thats why we do conversion training. I do find the Busses FMC a little more clever , however the Boeing did do what it was asked to do.Merry Christmas