Merge request

There is a question:
There are 2 branches, let’s call them Branch1 and Branch2, Branch2 is based on Branch1.
With the help of gitlab api, I create a new branch Branch1_new on the basis of the Branch1 branch and push there some changes.
Branch1_new in Branch2 (Changes section is full) changes appear in Branch2.
Branch1_new in Branch1 (the Changes section is filled) changes appear in Branch1.
But when I make a contractor 2 branch, branch 1, new in branch 1 and branch 2, that is, changes are available only in branch 1 (the Changes section is filled). No changes (Changes section is empty), why can not I understand?
Gitlab explain the reason