Use a trilinear sampler to filter across MIP levels

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumeVoxelization.compute

     float lod = ComputeTextureLOD(VxUV * _VolumeMaskDimensions.z, VyUV * _VolumeMaskDimensions.z, VzUV * _VolumeMaskDimensions.z);
     voxelCenterUV.z = clamp(voxelCenterUV.z, offset + clampBorder, offset + _VolumeMaskDimensions.x - clampBorder);    
     
-    float maskValue = SAMPLE_TEXTURE3D_LOD(_VolumeMaskAtlas, s_linear_clamp_sampler, voxelCenterUV, lod).a;
+    float maskValue = SAMPLE_TEXTURE3D_LOD(_VolumeMaskAtlas, s_trilinear_clamp_sampler, voxelCenterUV, lod).a;
 
     return maskValue;
 }