Well, title say is all. Iray mirror.mdl, when assigned to the materials (as well as mirror made with just roughness and metallic maps), would currently reflect IBL image (see attached slide)

I believe the inconsistency between IBL and SKY has been there for a while now.

I also believe this issue might directly relate to the MDL glass issue reported earlier:
https://www.reallusion.com/FeedBackTracker/Issue/iClone-glass-and-Iray-mdl-glass-see-through-mismatch