[Vuejs]-Vue: V-modeled number selector not reflecting changed value

You missed the underscore for the input

<input type="number" id="frameStart"  v-model="startFrame"/>

And for getter and setter

startFrame: {
    get(){
        return this.object.startFrame;
    },
    set(newFrame){
        this.object.startFrame = Math.max(Math.min(newFrame, this.object.frames.length - 1), 0);
    }
}