This is how the Pontils Tomb once looked.
The controversy comes from its comparison with the painting 'Shepherds of Arcadia' by Nicolas Poussin, which he painted between 1638 and 1639. He could only have painted the Pontils Tomb if a tomb, it does not have to be the one shown above, existed at the location when he was alive.

Nicolas Poussin
The Pontils Tomb that we know of today was constructed in 1903 by Bourrel, a stonemason from the nearby village of Rennes-le-Bains. The work was paid for by the grandson of M. Gilibert. It is assumed it was built as a family burial tomb, as in 1903 the remains of his grandmother were interred there, and later his wife Elizabeth was also placed beside her in the Tomb.
The surviving members of the Galibert family remained in Pontils for the next twenty years before moving to Limoux, a village situated a few kilometers away on the road to Carcasonne. There the family had a new tomb constructed in the Graveyard, utilising the facing stones and the metal gate from the Pontils Tomb. The two bodies from the Pontils tomb were then transferred to the one in Limoux.
Limoux graveyard.
The Pontils estate was then put up for sale and was duly purchased by Emily Rivares and her son, Louis Lawrence in 1921. Both were from America, though madame Rivares had French blood. A year later Marie Rivares, Louis's grandmother, died and was placed in the Pontils Tomb after being embalmed in accordance with her wishes. In 1931 or 1932 Louis's mother, Emily, died and she two was placed in the Tomb, apparently with the remains of her two mummified cats.

This photo was taken from this WEBSITE and is believed to be a photograph taken in 1972 of the interior of the Tomb.

Louis Lawrence 1884-1954

Pontil's Tomb - 1900's - More old photos of the Pontils Tomb HERE
This information was taken from the book, 'Histoire du Trésor de Rennes-le-Château’ by Pierre Jarnac. His sources for this information was the second son of Louis Lawrence. And the stonemason Bourrel who dug the first grave in 1903. It is unknown how reliable this information is.
So if the Pontils Tomb was only built in the1900's, it cannot be the same one that appears in the Poussin painting, Shepherds of Arcadia.
There is a rumor that the 1900's tomb was a copy of one that used to stand there some time earlier and which was destroyed by Colbert in his over zealous search for old mine entrances in the area.
Pierre Jarnac also reports that near to the Tomb was a mill, Moulin des Pontils - The Pontils Mill. It was this mill that the Galibert family purchased in 1880. At one time the mill would have been powered by a waterwheel, which were common place in the 1600's, so it is possible that a mill stood there during Poussin's time. The owners of the mill could have constructed a tomb in the same location as the later 1900's tomb. It was not unusual for such tombs to be built along the roadside, a few still remain in the area today, so it is not inconceivable that an older tomb, long since fallen to ruin, or like the Galibert family, a previous owner of the estate could have built a tomb that was then transported to another location when the family who owned it moved on, which seems to have been the done thing back then; families took the bodies of their loved ones and the expensive stone tombs with them.

The Old Pontils Mill Ruins