It isn't at all strange that the East Baltic languages developed (along with other PNIE languages) the neuter and then dropped it later. The very same happened

It isn't at all strange that the East Baltic languages developed (along with other PNIE languages) the neuter and then dropped it later. The very same happened in Albanian, Irish, and the Romance languages (except for Balkan Romance). Any recent introduction to IE linguistics starting from Beekes' Comparative Indo-European Linguistics will discuss the neuter being an innovation after the Anatolian languages broke off. However, I would especially recommend the Handbook of Comparative and Historical Indo-European Linguistics that will be published by Mouton de Gruyter this September, as it will contain a state-of-the-art survey of the field by a number of prominent scholars. See Lundvist & Yates' chapter on Morphology in it (you can also download a preprint PDF of this chapter from Yates' papers on Academia.edu if you have an account there).