Hi,
So here is what I found:
• Legacy and Physical material, with full refraction, no thin mode - they act the same, meaning objects which contribute to CESSENTIAL_Translucency render element are NOT visible through objects which have mentioned materials applied
• Legacy and Physical material, with full refraction, with Thin mode - they act different, meaning objects which contribute to CESSENTIAL_Translucency render element are visible through LegacyMtl and not through PhysicalMtl.
Is this exactly what you are referring to? The Thin mode option is the key here I think.