I assume that the following block defines a 3-line arrow function:

(): void => {   // Line 1
                // Line 2
}               // Line 3

I am demonstrating that max-func-body-length fails to count body length correctly. For instance, when you run npm run lint with this repository, lint says:

[20, 5]: Max arrow function body length exceeded - max: 2, actual: 3

Actually, however, the function has 4 lines.

This happens because it does not add 1 to endLine - startLine for calculation of body length.