disappear of add button in relationship panel for normal user

[Vincent]

Normal Users have the create right of Entity A, Entity B and
Entity A and B has an one to many relationship

In the detail view of A, there is a relationship panel of B
and it is very strange that only admin can see the + button in the relationship panel of B, but no + button for normal user.

how can I set the + button for normal user?

version: 7.0.10
Remark:
1. there is no such problem for relationship panel of other entities in version 7.0.10
2. there is no such problem for all relationship panel before upgrade from 5.1.2