the number of channels fixed in the sample function in spectral mode?

Summary

I'm debugging rendering in spectral mode in Python and have set

mi.set_variant('cuda_ad_rgb')

Initially, I configured a BSDF with 30-channel reflectance. However, during the debugging process, I noticed that the return value of bsdf_sample() only has 4 channels. Based on this, I also tried a 30-channel emitter, but the em_weight returned by sample_emitter() also has 4 channels.

This behavior differs from my original expectation. So, during rendering in spectral mode, is the number of channels in the sample function fixed at 4 channels, even if the material parameters may be set to have more channels?

System configuration

System information:

OS: Windows-10 CPU: Intel64 Family 6 Model 183 Stepping 1, GenuineIntel GPU: NVIDIA GeForce RTX 4090 Python: 3.10.14 | packaged by Anaconda, Inc. | (main, Mar 21 2024, 16:20:14) [MSC v.1916 64 bit (AMD64)] NVidia driver: 536.25 CUDA: 11.8.89 LLVM: 15.-1.-1

Dr.Jit: 0.4.4 Mitsuba: 3.5.0

Steps to reproduce

1.here is my test function

class real_data_integrator(mi.SamplingIntegrator):
    def __init__(self, props):
        super().__init__(props)      
    def sample(self, 
               scene: mi.Scene, 
               sampler:mi.Sampler, 
               ray: mi.RayDifferential3f, 
               medium: mi.Medium = None,
               active: bool = True) -> tuple[mi.Color3f, bool, list[float]]:

        L = mi.Spectrum(0.0)

        # ---------------------- Direct emission ----------------------
        si = scene.ray_intersect(ray, active)        

        bsdf_ctx = mi.BSDFContext()
        sample1 = sampler.next_1d()
        sample2 = sampler.next_2d()
        bsdf: mi.BSDF = si.bsdf()

        bsdf_sample, bsdf_weight = bsdf.sample(
                    bsdf_ctx, si, sample1, sample2)

        self.shape_confirm(bsdf_weight)

        valid_ray = active & si.is_valid()
        active &= si.is_valid()

        cosThetaI = mi.Frame3f.cos_theta(si.wi)
        active &= (cosThetaI>0)        

        # Sample the emitter
        ds, em_weight = scene.sample_emitter_direction(
            si, sampler.next_2d(), True, active
        )
        active &= ds.pdf != 0.0

        self.shape_confirm(em_weight)
        return dr.select(True, L, 0.0), valid_ray, []

    @dr.wrap(source='drjit', target='torch')
    def shape_confirm(self,em_weight):
        print(em_weight.shape)

mitsuba-renderer / mitsuba3