One royal fan account was quick to comment on the apparent snub: "Kensington Royal just posted a photo - and I thought it would be for Princess Diana's birthday. Sad."
Another said: "Happy Canada Day? Not happy birthday to Diana, Princess of Wales? Hmm. Poor taste, in my opinion."
A British user noted: "I've noticed they don't mention anything about Princess Diana's birthday - it must still be hard for them."
Others simply took the opportunity to send the first wife of Prince Charles a birthday message themselves, writing: "Happy heavenly birthday, Princess Diana - the People's Princess."
The lack of online tribute for Diana is highlighted by William and Harry's posts for other members of the royal family.
On June 10, the Duke of Cambridge and the Duke of Sussex marked their grandfather Prince Philip's 98th birthday with separate social media messages.
Earlier this year, Kate Middleton's 37th birthday was marked by a number of posts from Kensington Royal, Clarence House - the official page of the Prince of Wales and Camilla, Duchess of Cornwall - and the Royal Family's Instagram account.
However many users defended the royal siblings' right to honour their mother's special day in private.
WATCH to see why Prince Harry was hurt over the media's treatment of Diana:
"For all of those who say it's disrespectful or that they should post something for Diana's birthday: 1) they don't run this account, simple, and 2) don't you think William has his own way of celebrating (if he does) his late mother's birthday?" one person noted.
"Besides, it's different to post about someone who is alive (like Queen Elizabeth II, Prince Charles, etc.) and someone who isn't. It's not disrespectful and it's not your business."
Over the weekend, Kensington Palace announced the official split of the Royal Foundation, a joint charity founded by William and Harry in 2009.
The 'Fab Four' ran the Royal Foundation together, but Prince Harry and wife Meghan Markle, Duchess of Sussex, have reportedly cut ties with the foundation to pursue their own charitable interests, separate from the future King and the Duchess of Cambridge.
Royal correspondent Tom Sykes said: “It’s official: The Fab Four are no more. William and Kate will continue to oversee the Royal Foundation, while Harry and Meghan will set up their own charitable endeavour.”
But despite reports the Duke of Cambridge and the Duke of Sussex are drifting apart and the couples are embroiled in a royal feud, a Buckingham Palace insider reveals the decision to split has been in the process for some time.
It was seen as a “natural and organic progression” given that the couples have “very different ambitions”.
William and Kate will likely focus on British mental health and suicide prevention organisations while Harry and Meghan are said to be more interested in the Invictus Games and Harry's personal AIDS mission in Africa.