taylodge,
I’m sorry for the delay in getting back to you, I handed this off for someone to look into and hadn’t realized they were overloaded at the time and couldn’t get around to it.
I created a new sequence diagram with 2 objects. I then drew a single call from object 1 to 2. I then drew a return from the stems of object 2 back to 1 and it worked OK (I’ve attached the UMR I made, please have a look)
If I’m missing some step you are doing please let me know where I went wrong.