In my experience it could very well be the pivot point is very slightly worn at the pivot. It’s been quite a few years ago but I’ve had 2 beam scales rebuilt and recalibrated. One is an Ohaus triple beam that measures in grains. Probably bought it in the 70’s.
My memory says the guy I dealt with is in California. I also bought a rebuilt one from him - that’s what you do when you have boys and they move out and set up their own reloading benches.
I’m betting it’s your pivot point - top, bottom or both.
That pivot point should be Havalon blade thin.