Why are .ico thumbnails showing up tiny after the latest Windows 11 update?

Why did .ico files suddenly start showing tiny preview thumbnails in File Explorer after a recent Windows 11 update, even though the same icons display at the correct size when I use them for a folder or shortcut? The files themselves seem fine—only the preview is shrunken. I've already tried rebuilding the icon cache, but the thumbnails stay small.

Has anyone else run into this, and is there a way to get the previews back to their normal size without resetting the whole system?