Is there tension on the truss rod? I have had basses built with the neck built in such a way if I put any tension on the truss rod it would cause major buzzing on 1st, 2nd, 3rd frets so I let all the tension out of the truss rod and without any stabilization from the truss rod the neck moved all around. A neck must have some truss rod tension to make it as stable as it can be. I had to God forbid again put flats on the bass and tune it a half step up and leave it like that for over a month it pulled the neck forward enough and settled to where I could take those flats off and give a good snug full turn of the truss rod making it set up better and much more stable.
“If you are not building your dream then simply someone else has hired you to build theirs”