South African publication Rapport reported the duo stayed in a bed and breakfast in the week of the opening match against New Zealand.
One witness told Rapport there was "sounds of pleasure" emanating from the room, later replaced by a heated argument.
Jantjies, who is married, didn't face the All Blacks in the first two matches of the Rugby Championship but came off the bench in a 25-17 defeat to the Wallabies in Adelaide before missing selection in the next game.
Earlier this year he was charged with malicious damage to property and contravening aviation laws after allegedly breaking a TV screen and swearing at fellow passengers and crew members on a flight home from a holiday in Turkey, prosecutors said.
The World Cup winner was arrested in May when his flight landed at OR Tambo International Airport in Johannesburg. The charges were later withdrawn after he agreed to pay for the damage.
Jantjies was acting in an unruly manner on the flight, prosecution spokeswoman Phindi Mjonondwane said at the time. He was arrested when the plane landed after crew members complained about his behaviour.
During the flight, Jantjies was ordered back to his seat in the business class area after breaking a glass that cut his hand, Mjonondwane said. He is then said to have damaged the TV and a light.
Jantjies was a member of the Springboks' victorious Rugby World Cup squad in Japan in 2019, when he was backup flyhalf to Handré Pollard.
His agent, James Adams, said in a statement that a light was broken on the aircraft but the incident on the Emirates flight from Dubai to Johannesburg had been unnecessarily "heightened".
"Our client, however, remains compliant with the investigation process," Adams said.
Jantjies had been on a weeklong holiday with his family. He was flying home alone while they remained in Turkey, Adams said.