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.
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.
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.
(Reporting by John O'Brien in Singapore; Editing by Toby Davis)
