I asked chagpt the other day who in the middle ages was following Roman law/Codex Justinianus. It tried to dance around saying that the Italians kinda taught it in universities first being Bologna but then they also had canon at the same time, on top of that add feudal and clerical administration and it had to go up to Aquinas in 13th century to acknowledge "Natural Law" while at the same time stressing Canon law is superior to civil law. By the way there weren't any courts or anything like that either in the West, you were basically taking your case to your higher feudal appointee or magnate.

So that's the only answer you need, the Eastern Roman empire was the last standing Roman successor state that for all of its history had civil law and was the closest to republican civil administration with no feudal succession rights, while westerners worshiped and obeyed a man in funny hat who crowned an illiterate barbarian warlord.