I don’t have one of these but do have an older FEF, Challenger, and Big Boy with similar tenders. Take a look at the 5 axle fixed truck. The leading and trailing axles have shouldered axle bushings. The three middle axles should have bushings without shoulders to allow more side to side play. I have found at least one of these tenders to have shoulders on all the bushing and had to machine them off.
No idea if this is the issue here but worth a look.
Pete