This might be the same as the dark imagery that was mentioned in Unable to view high-resolution map in offline mode - #9 by HSimons
The likely reason is that the lighting computations are different for the imagery on the terrain, and the clipped model. When you drag the “time of day” slider at the bottom of a sandcastle window, you may notice that the geometry will (likely) be brighter when the sun is directly above it.
Depending on the exact context, you might disable the lighting computations with a custom shader. Is this glTF used as part of a tileset, or loaded as a standalone model?