Initial Value Issue in New Desktop Design

[James_Burns]

I have two tables, one called Accounts_Paid and another called Accounts_Payable. 
A field in Accounts_Paid is Accounts_Payable_IDs which is an EnumList type with base type of Ref to the Accounts_Payable table.

On the Accounts_Payable table the Vendor field has an initial value formula:
ANY(SELECT(Accounts_Paid[Vendor],
CONTAINS(SELECT(Accounts_Paid[Accounts_Payable_IDs],ISNOTBLANK([Accounts_Payable_IDs])),[_THISROW].[Accounts_Payable_ID])
))

In the form to create a new record in the Accounts_Paid table I can add new accounts payables to that record inline. The Accounts_Payable form that opens inline has the Vendor pre-populating to match the Accounts_Paid record as expected. However this does not happen in the new desktop design. For some reason in that format the Vendor pre-populates to the first one it finds alphabetically on the Accounts_Paid table.

Has anyone experienced this issue? Is this just a result of something I failed to do or is this a bug/limitation I need to raise with admins? Any help would be greatly appreciated!