I think it's more about his command when he gets kicked around. The entire wold knows his splitter is nearly unhittable when they can't identify it, but it's easier to identify if he cannot command the fastball down in the zone. If, when he throws the fastball and it's always upper half or a little wild, the hitters can basically let go anything they perceive to be lower half simply because they can reasonably guess it's the splitter and shouldn't swing as they know it will fall out of the zone far more often than not.
Here's yesterday's pitch 3d - red are the fastballs, the light greeny blue are the splitters, yellow are sliders.
Here's the same plot from June 6th against Houston, same colour on the pitches
Sure, there are still some fastballs up as you cant just exclusively not use the top half of the zone, especially when you can throw 98 with a heater, but there's a noticeable difference in alot of his fastball locations vs Houston than vs Minnesota.