Why doesn't this relationship structure work in Power BI?

Hi,

>>Where two levels have equal distribution of data, but the third level only references some fields from the previous tree structure?

According to your description, you can use the single direction relationship mode:

1. Click on “Manage Relationships”.
 

2. Click edit to edit the relationships.
 

3. Modify the both relationship to single.

Result:

Use this relationship, you can’t use “Record Details” to filter records which in “Records Header”, as the arrow points to “Record Details”.

Reference:

Create and manage relationships in Power BI Desktop

Regards,
Xiaoxin Sheng