There's nothing special about this. Let's review:

A Backdoor Roth is for someone who cannot contribute to Roth directly, so they make a Nondeductible Traditional IRA contribution, with the intent to Convert right away. As long as they have no basis in this or any other deferred account (per the IRS conversion table), then there is no income tax owed on a direct conversion. Because conversions are pro rata.

Not doing it timely is the same as having basis; there might be earnings that will be subject to taxes, now, because they were earned in a tax deferred account.