She doesn't say "casser". She says "gĆ¢cher". She doesn't have a French Quebec accent. It is pure Belgian, from her mom. I'm from Quebec (and lived all over the province) and nobody ever says "casser ta surprise". Elle is not even fluent in French. It is obvious in her vlog from Tahiti when she talks with the pearl lady. She speaks like someone who has never been exposed to the language, except from her mom. She can read Yves Saint Laurent labels and menus but that is about it.As native french from France I can confirm what you just said.
Elle's french is a mix of french Belgium, french Quebec, ( can't tell for the northe American valley girl) and Elle's preciosity.
In the video when she talks to her mom abt the Xmas present, she said : tu vas casser ( break/ ruin) ta surprise. Which is more Canadian.
French from France would say: tu vas gĆ¢cher ( spoil/ruin/waste) ta surprise.
Her baby voice doesn't help too.