Subnautica. I always have a lot of fun building out my base and discovering things and poking around, but after a while I get distracted by another game and put Subnautica on the back burner.
Another one is Darkest Dungeon. I have over 100 hours in that game and loved it, but I kind of stopped playing after all the strategies I had been using stopped working and all my parties kept getting annihilated. I was spending so much time recruiting, training, curing, etc that it sort of stopped being fun.