Go to a repo in BB -> Settings -> Set a "Main Branch " -> Save settings.
Go to the Branches on the Left -> Create Branch -> Branch From select any branch but the one that you set as a "Main Branch " -> Name your branch ->Create.
Now click on the Branch name you just created a see that the "Parent branch" for your created branch is actually showing as a Main Branch and not the one you selected as to branch from.

Expected behavior: to see the "Parent branch" as the branch name from which you branched out