It's been a year since I've done anything with actual inventory, but I remember having a similar issue while building the assembly item. If I went to the assembly item and chose to build assembly from the drop down actions page, none of the components bin locations would auto populate, but on that same page, if I removed the assembly item and added it back, the bins would auto populate. It was like a timing issue on NetSuite's side or something. We never had an issue with the actual final build assembly item not getting it's bin number set though. Sorry 😕