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No.
For example, the "Berlin Endgame" is technically an opening and basically skips the middlegame into an endgame. Openings with lots of opening theory tend to suffer from the same problem where basically most of the "middlegame" is still opening theory.
Many define an endgame as any position where the Queens are traded off the board and the King is more safe to start advancing towards the center without much fear of checkmate. However, this definition would make Queen endgames (like each side with one Queen and several pawns) part of the middlegame and not an endgame. There is no clear-cut definition.
If I had to define it the best I could, I'd say the middlegame transition to endgame is the stage of the game where there are less pieces on the board to a simplified extent that the players can begin thinking more concretely about endgame motifs and the King is in less danger of being checkmated compared to the middlegame, or opening. My impromptu definition is clearly lacking, but again, there is no objective way to define middlegame different from endgame really.
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