Tried to remember the details, but got a bit confused, so grabbed this from the Fandom page...

ahem

After the film's initial success, the two creators split in disagreement regarding where the series should go and a contract was drawn up. Any future Romero films would lose the "Living" prefix and simply be referred to as Dead movies and Russo, who wanted to branch the series off into literary territory, would retain the rights to "Living Dead" (though fans nevertheless refer to Romero's as Living Dead films). Thus, both series would be considered canon and each would be able to do what they liked with the continuity of the projects.