https://ask.usda.gov/s/article/Is-butter-safe-at-room-temperature

However, if butter is left out at room temperature for several days, the flavor can turn rancid so it's best to leave out whatever you can use within a day or two.

Nah man. I thought this. My butter went bad and I didn’t realize until I ate a lot of it. I was putting it on bagels I was getting and I kept thinking they were mixing like rye seeds in there. That was the closest approximation I could conceive of because I thought the same as you.

Nope. I kept eating sour butter. It was fucking disgusting. Keep your butter in the fridge.

The USDA literally says the opposite of this. It turns rancid.

But don’t let some rando on the internet be the judge. Cut a stick in half, leave half out, wait a week, then taste.