Let's say I make a feature branch Feature1 from master branch is currently on commit A. Then I make two commits in Feature1 branch B and C. Now I make a pull request PR1 (don't merge).
Then a branch Feature2 is done dependent on commit C, a commit D is done on this branch, and pull request PR2 is created.
Branches and pull requests picture:

Azure now displays everything correct on the two pull requests.
PR1:
Commits: Shows B and C -> correct
Files: Shows diff from A to C (Based on commit A) -> correct
PR2:
Commits: Shows B, C and D -> correct
Files: Shows correct diff from A to D (Based on commit A) -> correct
Now complete PR1 using fast forward.
The problem is now:
PR2:
Commits: Shows D -> correct
Files: Show still the diff from A to D (Based on commit A) -> **unexpected**
Expectation would be Azure should update the 'Files' diff and show only C to D (Based on commit C).
How can I tell Azure to change the 'Based on commit' pointing to C after complete PR1 on PR2? Of course I can choose 'View merge changes' from Menu, but in this case also the 'Files' tab should only show the merge changes in my expectation (only C to D).
I think this question is the same as here, but this question was not clearly asked / answered.