The hair may be knotted very loosely and if extreme caution is not taken when being washed, it could cause excessive unknotting of the hairs. From my limited experience, only someone who is an expert should be washing your wig. It needs to be washed while pinned onto a head and definitely no harsh brushing or combing when the hair is wet.
I have seen in some beauty salons that the wigs are washed by the hired help who work at minimum wage. They use inferior shampoos and conditioners and use excessive force when handling the wigs.
In my opinion, unless the person who does the washing has any sort of knowledge of the value of what they are handling and that extra TLC needs to be given to an expensive wig, they are not handling it properly. An actual sheitel macher who washes a wig knows that if she won’t take proper care of the wig that she will end up having to deal with the repercussions. I would tend to believe that the minimum wage worker couldn’t care less.
I hope the salon who did the washing will resolve your issue satisfactorily.
Good luck!