I like to see how far I can push the memory and don't use xmp/expo.The problem with pushing ram like this is in the real world it makes no difference at all.
Are you manually setting the timings?
If not the main ones set by xmp/expo will stay the same but the others will auto relax to accommodate the higher speed which can result in slower ram
This particular set runs 8400 cl36 lga 1700 and 9066 cl42 lga 1851 just fine.