Textures with source checks from image if it has transparency (see QSSGLoadedTexture::scanForTransparency())

Textures with sourceItem don't have this option. To have blending user needs to set blendMode to something else than sourceOver (see QSSGLayerRenderPreparationData::prepareDefaultMaterialForRender() where renderableFlags gets HasTransparency when blendMode != SourceOver). But other blendModes cause effects which user may not want, items with SourceOver should also have transparency.