Japan were fielding a weakened side due to Super Rugby commitments for a majority of their big-name players but a plethora of handling errors by the favourites helped Hong Kong keep the scoreline down to a respectable total.
The visitors led 3-0 at halftime before Lotoahea scored in the 47th minute after he had been put clear by centre Kanta Shikao. Flyhalf Takuya Yamasawa kicked a successful conversion to go with his three penalties to complete the scoring.
The champions defeated Hong Kong in Tokyo last week after recording home-and-away victories over South Korea earlier in the three-team tournament.
At last week's draw for the 2019 Rugby World Cup, Japan were placed in Pool A alongside Ireland, Scotland and two yet-to-be determined qualifiers.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
(Reporting by John O'Brien in Singapore; Editing by Toby Davis)
