I've noticed this too. I think either there was a sneaky patch to the cyrptarch packages from the postmaster because I used to get at least 1 legendary everytime. Now, obviously with the patch that means purple engrams now equal purple items and the fact that you can get a shitload of blue engrams from strikes to rank cyrptarch up superfine, I think they have lowered the rate that we obtain purples from the packages otherwise it would be too easy to farm shards etc. Either that or I'm having extremely bad luck with RNG.