Does the ruling on visiting paternal uncles differ from that on visiting grandparents, such that visiting a paternal uncle after a month is not considered abandonment, while visiting a grandparent after a month is considered abandonment?

There is no harm in visiting grandparents more than visiting paternal uncles, as the grandfather takes precedence over the uncle in maintaining kinship ties, especially if custom dictates it. Indeed, the maintenance of kinship ties refers back to custom.