This was addressed in the TNG episode Where No One Has Gone Before. It would appear that the Enterprise has (at least for the first few episodes) multiple Chief Engineers. Later on, Geordi seems to have been made the sole Chief Engineer with various Assistant Chief Engineers under his command.
KOSINSKI: A ship's engines should be a concern of the ship's captain...
RIKER: They are, sir. Which is why they make their first officers directly responsible for engine condition and performance.
Guided, of course, by one of our chief engineers, Lieutenant Commander Argyle in this case.
Out-of-universe the show had staffing issues and couldn't seem to settle on a permanent actor for the role until the decision was taken to bulk out LeVar Burton's character.
On a related note, it might interest you to learn that the ST Encyclopedia identifies Lt Cdr Lynch as Assistant Chief Engineer.
And Argyle as a Chief Engineer, lending credence to the idea that in the early period of the ship's operation that there were multiple people with that title