However, other royal fans defended William and Kate for the post and called out the trolls for jumping to conclusions.
"Hey haters - they probably picked it because a bunch of them are together laughing," wrote one user.
Fans of Will and Kate also praised the royals for presenting a united front.
"I like this demonstration of unity and family. So important," said one touched user.
Meghan has also received birthday wishes from other royal family members including Queen Elizabeth, Prince Charles and Camilla.
British Vogue Editor in Chief Edward Enniful and her best friend Jessica Mulroney also sent the Duchess birthday wishes.
Prince Harry also took to Instagram to wish his wife and mother of his child Archie Harrison a happy birthday.
Last month, Meghan and Kate put the rumours of a royal feud to bed after they were seen smiling together at Wimbledon.
According to reports, the two have only grown closer since Meghan and Harry's wedding.
A source told Hello that "Kate was a great source of support to Meghan in the days leading up to Archie's birth and, despite reports of a rift, they are family and have a really lovely friendship."
The source also said Meghan and Kate "talk and text regularly," and have more in common since Archie was born.
"Kate has wanted to make sure Meghan has felt welcome into the family," the source explained.