Since Patch 1.52.100 there’s an option for Vampires and Mermaids to link both forms together. Linking them is the default.
Because of this change it seems that you can’t change the forms individually anymore more and that the connection is fixed so you can’t change one without the other.
Once linked all parts that you have clicked will be changed together automatically. You can also copy over certain parts from one form to the other. Like with the linking only the parts you have chosen will be copied.
To get rid of the link, follow these step
Click on the chain icon to link or unlink them