My brother went through this all the time with his kids BM. He would buy new clothes and they would always disappear.
Mom Dukes told my brother she’s doing 1 or 2 things. She’s taking them back to refund them to get the money or she’s trying to antagonize you into doing something dumb.
Finally, mom dukes told him when she brings him over with worn out/dirty clothes take them off and put on the new clothes you bought him and when she picks him up or you drop him back off take off the new clothes you bought him and put him back in the dirty clothes she had him in.
When she asks why he’s in dirty clothes tell her you put him back in the clothes she brought him in and that he you’re not sending him back with clean clothes that never get seen again.
She never brought him over with dirty clothes again.