The sentence “I saw he” contains a syntactic mistake.
2001, Martin Haspelmath, Language Typology and Language Universals: An International Handbook, page 674:
the rules specifying how agglutinative morphemes are combined with each other are more syntactic than morphological by their nature and thus are closer to rules specifying how word-forms are combined with each other.
Containing morphemes that are combined in the same order as they would be if they were separate words e.g. (m)