Merge request isn`t marked as "merged", if the source branch was deleted before the merge commit was pushed

Summary

Merge request is not marked as "merged", if the sourse branch was deleted before the merge commit was pushed.
The issue did not take place several releases ago.

Steps to reproduce

On local machine:

1. Merge source branch into taget branch (as set in merge reques);
2. Delete source branch locally and on the origin (gitlab server);
3. Push target branch with merge commit.

What is the current bug behavior?

On merge request page we sww the following: "Source branch feature/... does not exist
Please restore the source branch or close this merge request and open a new merge request with a different source branch."
As the result - we have nothing to merge, and can`t see that the request has already been accepted.

What is the expected correct behavior?

We expect that the merge request is marked as "merged".