I don't think it's so clear cut. The wikipedia article cites a 1971 book and an article from 1991 as sources. A paper from 2014 is not so sure this is a settle

I don't think it's so clear cut. The wikipedia article cites a 1971 book and an article from 1991 as sources. A paper from 2014 is not so sure this is a settled matter: "Infant object permanence is still an enigma after four decades of research. Is it an innate endowment, a developmental attainment, or an abstract idea not attributable to non-verbal infants?" and argues that object permanence is not in fact innate but developed: "It is argued that object permanence is not innately specified, but develops. The theory proposed here is that object permanence is an attainment that grows from a developmentally prior understanding of object identity" "In sum, it is posited that permanence is initially dependent on the nature of the occlusion; with development, it becomes a property of objects. Even for 10–12-month-olds, object permanence is still a work-in-progress, manifested on one disappearance transformation but not another." https://pmc.ncbi.nlm.nih.gov/articles/PMC4215949/ https://pmc.ncbi.nlm.nih.gov/articles/PMC4215949/