Once the nodes are loaded from the NodeSrc, they are added to the Nodes collection of treeview instance and thus they do not refresh every postback again.
ForceInit property just forces the FlyTreeView client-side runtime to complete client-side initialization of loaded nodes in order to let developer to use client-side model in full.
Probably you just need to clear the nodes collection every postback, thus your nodes will be loaded every postback from the NodeSrc.