Death Siphon critical hits don't heal for 150% of the damage done from critical strikes
Death Siphon always heals for normal 150% damage from regular hit no crit even if the spell did crit the heal did not
let's say you crit for 35k and it only heals you for 22k that's not 150% heal from damage done
normal hits heal for 150% crit doesn't
5.4.8 patch