Daniel Abt says he became "completely unbuckled" after swapping to his second Audi Sport Formula E car during Saturday's race in Punta del Este.
Abt, who won the previous race in Mexico City after a rapid mid-race car swap, was running third when the seatbelt issue occurred at the Uruguayan coastal circuit.
He came into the pits to get the belts fixed and eventually came home 14th.
"I was completely unbuckled in the car, which I have no clue how it's even possible," he said.
"This we have to figure out, but of course in that situation what do you do? You risk your life or you come in - for me it's no choice.
"So I came in, redid the belts, and then we lost too much to gain any points."
The incident was investigated after the race, but the stewards were satisfied that "after hearing the driver and the team manager and the deputy technical delegate, the stewards consider the driver left the pits after mandatory car change with buckled and tightened seat belts" according to an FIA bulletin.
The data of the belt tension device on Abt's two shoulder straps showed that after the pit stop his belts were fixed properly and no further action was taken.
FE's car swaps have been under intense scrutiny in recent races after the minimum pitstop time was removed ahead of the Santiago event.
The FIA had to clarify the issue following a request from the teams and an incident in Mexico where Andre Lotterer clipped a mechanic as he exited the Techeetah pits.
"[It was] a real shame," said Abt.
"It was a very good race, I felt very strong, managed to overtake [Oliver] Turvey and then [Alex] Lynn before the pitstop.
"I came in P3, went out P3 - all good, feeling really fast - and then after two laps I realised that my belts basically came loose under braking - all four."
Audi team principal Allan McNish thought the issue cost the team a double podium, as Lucas di Grassi finished second, just behind race winner Jean-Eric Vergne.
Di Grassi was also investigated after the race and was fined for wearing non-compliant fireproof underwear.
"Daniel was looking for a podium as well," McNish told Autosport.
"He went out - it was fine - then they got loose somehow and then he clearly came straight back in to fix them and off he went again and that was it.
"I've had that personally before in other championships and you know when it happens and you have to come straight back in. There's no question about it.
"We have to find out what happened.
"He abided by the rules at the end of the day - they checked and it was correctly connected and everything else going out - there's no question there.
"But afterwards he came straight back in. He had to do it."