Okay, I've hunted around on this one a bit, but still not certain of the answer on how the devouring mechanic plays out with the Tendril.
It's pretty clear that the Ambush mechanic will kick in if the Tendril is not immediately evaded on the first round, and that the investigator is devoured if they still have sanity and stamina after the second round.
If the investigator is knocked unconscious or driven insane before the 2nd round, are they devoured or sent to St. Mary's/Arkham Asylum? Here's what I found on BGG, but still not sure if there is an official ruling on this: