The addition of "xo", the shorthand term for a kiss and hug, has led many commentators to speculate that former Suits actress Meghan is behind this and many of the other posts from Sussex Royal.
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But eagle-eyed royal fan watchers were less than impressed with how the Sussexes referred to their tiny royal relative and hit back at the couple for failing to use the Prince's official title.
"@sussexroyal if you don't believe in titles that's fair enough, but then don't expect people to use yours to address you," one user wrote, per Express.co.uk.
Another said: "It's PRINCE Louis to you. He outranks you both!"
Prince Louis is fifth in line to the throne, after his grandfather Prince Charles, dad William, brother Prince George and sister Princess Charlotte.
But while some users slated the Duke and Duchess for "taking the attention away" from the youngest Cambridge child, others rushed to their defence.
"Get over it - I wouldn't call my own nephew Prince!" one defiant user said.