Just a theory on why the cyanide had no effect.
He may have suspected some attempt on his life and as a precaution he could have eaten a couple daffodil bulbs.
Daffodil bulbs contain atropine, the antidote for cyanide and many nerve agents. Atropine via daffodil bulbs will give you an upset stomach but won’t kill you.
Cyanid is not a neurotoxin, and atropine would not be an antidote. It prevents aerobic metabolism, killing through hypoxia.
Rasputin also was probably a user of Amanita Muscaria
often found at the foot of birch trees on the taiga
and used by rural shamans, so his tolerance for alkaloids
may have been pretty high.