If a piece of land has an easement such as a right-of-way, and that piece of land subsequently gets divided, then the easement (right-of-way in your case), continues benefits of both pieces after the land was divided. The reason for that is that it’s not possible to say which part of the undivided land had the benefit of the easement and therefore it applies to the whole piece of land and hence to the divided parts.
If someone is telling you otherwise, they are wrong.
There is an exception and that is that if the right of way enjoyed by your father is personal to him (generally if he is named in the deed creating the right of way), then it would only apply to his piece of land. This latter situation is unusual.
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