And things didn't improve once the pair exited the car.
The usually polished Charles was seen turning back to Camilla - who was trailing several steps behind - apparently sending some terse words her way.
According to onlookers, the apparent confrontation was over in less than a minute, and Charles and Camilla were soon all smiles for the flashbulbs in the press pit.
But for our royal insiders, it was a "very telling" moment.
"They are usually totally united," says a royal watcher.
"Charles can be a fusspot, but Camilla often escapes his mini-tantrums. It seems even she's not immune now. There is a lot of pressure on them."
It's widely understood that Camilla has been suffering from back pain for the most of this year, and was seen grimacing in May when the heavy crown was placed on her head during the coronation.
Indeed, in France, she was seen walking gingerly down the stairs of their jet, and at one point Brigitte appeared to offer her a steadying hand.
One source says the tour has taken its toll on the Queen - and she's beginning to dread future large-scale engagements.
"It takes a lot more out of them to do these trips, especially Camilla," says a source.
"She hates to make a fuss about it, but her back problems are real. It's hard for her to stand for so long and she's nervous around steps. It's enough to have anyone at breaking point."
A separate source adds, "Charles is very businesslike on tour and gets stressed if anything holds them up. It's not making things easy."
Unfortunately for the couple, there's no chance of slowing their schedule in the coming years despite their advancing ages.
It's understood that the UK Foreign Office is under pressure to plan more royal tours to far-flung Commonwealth realms.
However, with the Prince and Princess of Wales raising a young family, and the Duke and Duchess of Sussex sidelines in California, Camilla is "all too aware" that the pressure rests on her seemingly fragile shoulders.