My experience is that Nintendo doesn't care much about such issues and will claim that you didn't properly press the button when detaching the joycon.
But check if the button properly veers back after pressing it.
Sometimes it's just dirt in the button that prevents it from returning to its original position.
A spraycan with air can sometimes help but sometimes you need to open the controller for some cleaning with alcohol.
On Chinese marketplaces they sell replacement buttons for 2~3 pounds so that would also be an option.
Don't rip the flat cables when trying to open the joycon.
They can be pretty fragile.
Also check the bottom of the joycon rail to see if there might be some dust in there.