Just wanted to say that I had the same issue and also fixed it by manually allocating my labels to TRAIN, VALIDATION or TEST, rather than relying on UNASSIGNED to do it for me.
Not sure if I’m misunderstanding the documentation—perhaps TFLite’s CSV loader is linking to the AutoML documentation because it uses the same spec but just doesn’t support the UNASSIGNED feature? The docs do say. ‘assigned to one of the above three sets by AutoML Vision Object Detection’ (emphasis mine). If this is indeed the case, perhaps the link from TFLite should note this qualification.