Why would a grammar site say that the accusative of solum ( neuter) is solud? does that word even exist?
ipsud was a neuter nom. = acc. byform in line with the regularity you mention. This uncertainty in the choice of pronominal endings is why you have ille, illud but ipse, ipsum - alongside ipsus, ipsum like tōtus, tōtum. This extends to the Genitive: -ī̆us vs -ī/ae, later even augmented as illaeī (> It. lei).But I don't know if or why anyone back then would ever have gotten it into their heads to make the accusative different from the nominative.