It seems like the tiles that are generated with FME all have this issue of the metallicFactor being 1, although it should probably be 0 in most cases. The same question came up a while ago, and I posted a workaround that can change the metallicFactor in all GLBs in B3DMs: 3D-tiles get very dark textures - #17 by Marco13
Note that this is really a workaround, and not part of any functionality that is officially supported.
But considering that it already came up twice, I opened an issue to track it at Consider a command to change the metallic value of GLBs in B3DM · Issue #65 · CesiumGS/3d-tiles-tools · GitHub . Maybe we can add a functionality to the 3D Tiles tools that can reliably fix these FME tilesets.