git rebase does not rebase commits

Hello Git team,

If I am trying to rebase commits from one unrelated history to another but this does not work.
Example:
* Merge
|\
| *x1 (test)
| *x2
*y1 (upstream)
*y2

git rebase --onto upstream upstream test

The TODO label onto

# Branch openapi3-exception-handling reset [new root] pick x1 pick x2

I think problem is here because 'test' branch does not have fork point with 'upstream'

Expected: label onto

# Branch openapi3-exception-handling reset upstream pick x1 pick x2

Is this a bug or I did something wrong or missed some options?