Try to Generate UVs for imported object from ilustrator... then place decal.
Adobe Dimension uses polygon-based mesh structures for its 3D models. Here's a breakdown of how mesh structures work in Dimension and what kinds are supported: May be that can be a reason.
Mesh Structure in Adobe Dimension
Polygon Geometry Only: Dimension supports only polygonal meshes. It does not support NURBS or curve-based geometry.
Editable Basic Shapes: You can use built-in customizable shapes like cubes, spheres, cones, and cylinders to build geometric structures. These can be resized and combined to form more complex compositions.
Imported Meshes: You can import external 3D models in formats like .OBJ , .FBX , .STL , .GLTF , .GLB , .SKP , and .USD . These models must contain polygon geometry to be compatible.
Native Format (.DN): Adobe Dimension’s native format preserves mesh details, materials, and decals for reuse across projects.
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