In the past, Toobworld Central has sent raiding parties across the border into the "Cineverse" to kidnap certain movies and claim them to be part of the Toobworld Dynamic.
The first "Highlander" movie
The "Star Trek" franchise
Sometimes, only one character from a movie crosses over into the TV universe to join the counterparts to those left behind - Radar O'Reilly of 'M*A*S*H' for example.
Neither situation applies in the case of Hercule Poirot, as played by Sir Peter Ustinov.
Why would Earth Prime-Time have any need for Ustinov's Poirot when it already had David Suchet as the little Belgian? (And who is about to complete filming on the entire series of stories featuring the detective. Not even Jeremy Brett was able to accomplish that with the Sherlock Holmes canon.)
But there must be an alternate TV dimension whose boundaries with the movie universe are not defined, where there is a blend at certain junctures so that characters can exist in both at the same time.
Such would be the world in which we would find Hercule Poirot as played by Ustinov.
Ustinov played Poirot sixt times - in three theatrical films
"Death On The Nile"
"Evil Under The Sun"
"Appointment With Death"
and in three TV movies.
"Thirteen At Dinner"
"Murder In Three Acts"
"Dead Man's Folly"
I'm not prepared to say that Ustinov was playing two different Poirots. Rather, I think there could a more fanciful splainin and Occam's Razor be bleeped!
Perhaps Hercule Poirot of this "Borderlands" dimension found a way to extend his mortal life. He may have paid a visit to a certain Sussex "scientist" and beekeeper for the secret benefits of the Queen Bee's royal jelly......