This is a good point, and maybe that is the design intent, to give more incentive to run missions with non-boxed groups.
Dev Ngreth posted in the Live Update Bugs 12/19/17 Thread what the drop rates are for visibles: T1 = 50% from a named (100% from chests) and T2 = 20% from a named (100% from chests). So it does appear like there is a strong incentive to get a real group together to go after those mission chests.
My hunter archievements say i killed 12 T1 nameds and 19 T2 nameds so far. I got 1 T1 helmet and 1 T2 bracer as visibles from nameds. So RNG hates me. dablubb
And I looked at the date on my changelog. It is still 20% live. it would have been 50% on the patch. I see the argument and will up it some more, but still not 100%. So it will be 75% on the patch.