Consider this test:

const original = `interface Test {
    a: boolean
    b: boolean
    c: boolean
    d: boolean
    e: boolean
    f: boolean
}`

const incoming = `interface Test {
    // a
    a: boolean
    // b
    b: boolean
    // c
    c: boolean
    // d
    d: boolean
    // d
    e: boolean
    /**
     * multi-line comment
     */
    f: boolean
}`

it('correctly determines the indentation to be 4 characters for both', () => {
    const originalIndent = detectIndent(original)
    const incomingIndent = detectIndent(incoming)
    expect(incomingIndent.amount).toBe(originalIndent.amount)
    // FAIL:
    // originalIndent.amount === 4 (correct)
    // incomingIndent.amount === 1 (incorrect, seems to use the multi-line comment indent)
})

I would expect that the incoming indent would be 4. The multi-line comment means that we return 1 as the indent amount