I have done similar ( or maybe worse) - gone to NRT on a one-way ticket, then started
a RTW from there, returning much later.

As long as you show that you have a ticket to depart within the visa-exempt period,
there will be no issues whatsoever.

And likely the only person asking about it will be the check-in agent for your first
flight; as long as you show them you have a reservation to depart, there will be no issue.