Why doesn't gitlab see the merge already done?

Question

GitLab

Hello.
I found a rake, and I can’t get off them :(
In short, there is gitlab and a turnip.
There are two branches.
I make changes in branch A, then transfer them to branch B via merge request . I
press merge.
Colleagues add their changes.
Then I I make another commit, and merge request.
And at this moment I see that the last diff includes not only the changes that I just made in branch A, but also the changes from my previous commit.
How can this be? that gitlab does not see the previous merge request at all (and the merged mark is definitely on it), although there are changes in the files, that is, merge happened.