For some reason as soon as I get to the “end” of the game, I completely lose motivation to play. This happened with DOOM: Eternal, Sekiro, Bloodborne, Paper Mario TTYD, Cassette Beasts, Shovel Knight, Oblivion, Baldur’s Gate, God of War, Mass Effect… Pretty much every game I’ve ever played. It’s like the fun part was the journey, and I’ve seen everything the game has to offer, and I just don’t really enjoy the ending process. Feels like a formality at that point.
It depends on the game for me, but it’s happened a couple times. Usually with metroidvanias, my favourite part of those games is the exploration/secrets, so if I’m confident there’s nothing left to find I sometimes lose interest. It also depends on my play schedule. Of I finish off a session right before the last boss, and then I have an opportunity to game for a few hours next time, I’m less likely to pick it up again just to finish it off. At that point I’d rather jump into a new game and start getting immersed in a new world.