The standard thumb strap on an Ariette is too short and rough. Find some soft leather at a craft shop and make one that's comfortable. I've had to do that on several accordions.
I've always had trouble with thumb straps.. no matter the length or material.
I solved it by not using one and playing with my thumb on the edge of the fingerboard. Many makers build with a thumb groove.. many players have added one.
HI
I switched my thumb strap using velcro.. I use the soft fuzzy side facing to my skin the other side well I just sits there.. The nice thing is, Velcro don't stretch being made of nylon or whatever its made of.. Very strong stable stuff.. Works for me anyway so far..