What version are you using?
v4.9.0

What did you do?

Config:

{
    "env": {
        "es6": true,
        "node": true
    },
    "extends": "eslint:recommended",
    "parserOptions": {
        "sourceType": "module"
    },
    "rules": {
        "no-fallthrough": [
            "error",
            {
                "commentPattern": "falls?\\s?through|break"
            }
        ]
    }
}

Code:

const item = "Test string";
switch (typeof item) {
	case "string": {
		// Run as if it was a number (below)
		// fallthrough
	}
	case "number": {
		console.log(item);
		break;
	}
}

What happened?

Console (worked as intended):

Test string

ESLint:

Expected a 'break' statement before 'case'. (no-fallthrough)

What did you expect to happen?

The same as without curly brackets, right? Removing the first or all of the case brackets prevents ESLint from erroring out the no-fallthrough message.

Code:

const item = "Test string";
switch (typeof item) {
	case "string":
		// Run as if it was a number (below)
		// fallthrough
	case "number": 
		console.log(item);
		break;
}

The empty block looks good for consistency especially since I want to comment something and not "stack" the case statements on top of each other.