Relationships not listed between entities when changing stage in Business Process Flow

Hi Guys,

Weird problem here, we have just moved to Dynamics 365 and I have a Business Process Flow against a custom entity.  I have added a second stage targeting the Account Entity but the relationship drop down isn't listing any of the available relationships:

here are the three relationships I expected to see

If I set the BPF to be against Account and set the second stage to be the Custom Entity they appear in the list.

Am I doing something stupid or is this a limitation of the BPF?

Thanks Stu.