I really don't understand how they can just say "we refund you and you'll have to go through the process of ordering online again", surely they could have had a manager keeping your name on a waiting list and allocating an XL for you right when they get stock.
Growing up in Brittany (i.e. France, but I always prefer saying I'm Breton rather than French

) I've known Decathlon for absolute ages and have bought a lot from them, always very pleased with Decathlon but this online ordering experience really got me to see them differently. Can't blame the shops and the staff there, they are great, but there definitively are logistics issues here, and COVID does not justify nor excuse these.