The 2017 St. Louis Cardinals entered the season with what seemed like a fairly strong closer to rack up saves throughout the year. Seung Hwan Oh, initially a run-of-the-mill reliever without the shiny closer designation when his Cardinals career began in 2016, took over when incumbent closer Trevor Rosenthal had easily his worst professional season. And in 2017, when Oh began to falter, Rosenthal took the role over again and pitched more like the excellent reliever he was from 2012 through 2015 rather than the disaster he was in 2016.
Seung Hwan Oh and Trevor Rosenthal were volatile relievers from one year to the next, but this is hardly unique among MLB relievers.