The numbers I referenced from Jansen ran from early May of 2021 until the end of 2022, that's nearly two full MLB seasons. I also referenced the fact that Jansen only played about a full season's worth of games for a typical starting catcher during this time due to injuries. It's certainly possible he would experience cold stretches if he were to remain healthy more often and experience the typical accumulation of fatigue of a typical MLB regular, however he managed to maintain this type of production over several seasons, despite a ton of stops and starts due to trips to the injured list. He maintained excellent peripherals, barrel rate, plate discipline etc. which to me suggests this wasn't entirely an unsustainable fluke run of success.
I was curious what's lead to the lack of success this season, and one thing that stands out is inability to get balls into the air to the pull side. He has hit a larger percentage of pull side fly balls out of the park compared to 2022, however far too frequently his batted balls to the pull side have been on the ground. Another change is that Jansen has been hitting the ball more frequently to right field with awful results to show for it. Hopefully this isn't a conscious choice as he was easily at his most productive when pulling the ball in the air.
Kirk managed to stay healthy for the entire season of 2022, but it's not out of the question that he wore down as the season progressed due to how much his overall production reduced as the season went on. I'm not one to rag on the guy at all as I think he's almost the perfect backup catcher who's bat is so good that he can regularly DH on days when he's not catching, and while he is catching he is a great defender. I just won't ignore the fact that his build greatly limits how frequently he is available to catch.