Yes, it seems so. While I don't remember the tuesday club (where Joyce appears) being mentioned in later books, bot Joyce and Joan are partners of Raymond and artists, so it makes sense that they are the same, and the name switch was a mistake (It happens occasionally in other authors' books - I mentioned here that in Patricia Wentworth's "Miss Silver" series the servant starts as Emma Meadows, becomes Hannah Meadows and then switches back - in one book she has one name at the beginning and the other at the end!
Comments
Yes Joan and Joyce are the same person, The Thirteen Problems is s Brilliant Book, I love it, most of the short stories are excellent.