I’m just a beginner with UE5 and Blueprints but I think I found the problem. In the CreateMaterial function of the HDRIBackdrop blueprint, the two dynamic instance materials for the floor and sky are created with the same name HDRI_MID so I just changed them to HDRI_FLOOR and HDRI_SKY and it seems to have fixed the problem. I noticed that before the fix, the HDRIBackdrop Geometry component in my scene had the name HDRI_MID for the floor material and MaterialInstanceDynamic_413 for the sky so maybe this auto generated name caused an issue during packing.