Even if you think you’re doing everything right, you’re not. Well, the worst place to keep butter is in a butter dish, and the worst place to keep the butter dish is in the “butter keeper” of your refrigerator.
Butter dishes were invented to facilitate serving, not preserving. Because they’re not airtight, the butter’s surface is exposed to air and can oxidize and become rancid.
Butter compartments should be banned. Many of them have little heaters inside to keep the butter at a slightly warmer temperature than the rest of the fridge to make it easier to spread. But the warmer temperature speeds up oxidation of the fat.
I keep my butter in the freezer compartment, tightly enveloped in plastic wrap. Yes, it’s hard as a rock when I want to use it, but a sharp knife can whack off a piece that will warm up and soften rather quickly.