I've just discovered that the latest versions of the struct example code (Struct_send.ino,Struct_receive.ino) in the GitHub repo contain different encryption keys.
Receive:
#define KEY "thisIsEncryptKey"
Send:
#define KEY "sampleEncryptKey"
At a guess, this shouldn't be the case? I remember them being the same so it took me a while to figure out why it wasn't working!
Fixed, thanks!