That's not a fact. That's an assumption.
We throw metrics and hitting and pitching and can measure when they change. ERA goes up. OPS or whatever goes down. We know something has changed. THe results are now different.
But defense seems to live in this netherworld where fans think that because there is no accurate way to track it and measure, it must never change. If a player can play good defense today, then he clearly will tomorrow and every game for the rest of his career. But it is simply flat out not true. Disregarding obvious exceptions, like the Yips, players can simply have defensive slumps. They can have good games, bad games, good stretches, bad stretches. Some of it physical, mental, luck-oriented, and sometimes flat out physical ,like when the wear and team of playing 130 previous games makes an already sore arm throw a little softer.
We've seen Devers start out slowly on defense, and gradually get going multiple times. We've all seen other players just have days when they have some flat out issues, like Verdugo dropping a mildly tricky flyball from Nelson Cruz. And then go on to misplay a wall ball from Austin Meadows for 4 innings later. Is Verdugo a bad outfielder? Or did he have a bad game? I've seen him make plenty of catches more difficult than that Cruz flyball.
There is no real reason to assume defense just stays the same, just like you don't with hitting and pitching...