Hi Archetype,
This involves three entities according to your description, they are Account, Order and Order Line, and they have following OOB relationship:
Account and Order is 1:N relationship
Order and Order Line 1:N relationship
So Account and Order Line has no direct relationship, what you need is that display grandchildren records(Order Line) in subgrid of grandparent’s(Account) form, Order entity is necessary.
1.New 1:N relationship between Account and Order Line.
(1) Create relationship and add the lookup field to the order line form.
Go Settings > Customizations > Customize the system > Entities > Account > 1:N relationships.

Go Settings > Customizations > Customize the system > Entities > Order line > Forms > Information

(2) Create order line sub-grid in Account form.

(3) Fill order account with customer value from order.
The step is necessary, it missing step is why your sub-grid are missing data, because your lookup field is empty.

(4) Check sub-grid in Account form.
2.Nested Editable Grids
Nested Editable Grids in D365 CE v9 UCI | D365 Demystified
(1) Go Settings > Customizations > Customize the system > Entities > Account > Forms to select one form.
(2) Add order sub-grid in the form.

(3)Configure Nested Editable Grid.

(4)Test.
Expand one order, you can view related order line records.

Regards,
Leah Ju
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