SPOILERS inbound for TENET movie:
By the end of the movie, it is revealed that Neil and the Protagonist were friends for a long time in the "future" and Neil was on his way to his death to help the Protagonist in the "present".
Since Neil's death doesn't apparently affect his activities in the future (Grandfather Paradox maybe?), does it also mean that earlier in the movie, when the Protagonist dies by eating the cyanide, did he actually die at that point itself?
Point to consider: At the end, Ives asks the other two to die at their own will and not reveal the source of the separated algorithm, so since the Protagonist was interrogated at the time, did he die at his own free will?
TLDR: Did the Protagonist dying by cyanide in the initial stage of the movie his actual death?