Wrong inline changes in the Changes tab of some Merge Requests

Summary

For some Merge Requests, changes represented in the Changes tab are not adequate. For example, I'm looking at commits in the Commits tab and I confirm they are adequate, but some of the changes I see in the Changes tab should not be there as they already are in the target branch (and not even in the commits from the Commits tab).

It's like sometimes the "diff" is produced with the project default branch (master in my example) instead of the MR target branch (develop in my example). Any idea? Thanks.

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System information
System:         Ubuntu 16.04
Current User:   git
Using RVM:      no
Ruby Version:   2.4.4p296
Gem Version:    2.7.6
Bundler Version:1.16.2
Rake Version:   12.3.1
Redis Version:  3.2.11
Git Version:    2.18.0
Sidekiq Version:5.1.3
Go Version:     unknown
GitLab information
Version:        11.2.3
Revision:       06cbee3
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://git.foobar.com
HTTP Clone URL: https://git.foobar.com/some-group/some-project.git
SSH Clone URL:  git@git.foobar.com:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: no
GitLab Shell
Version:        8.1.1
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks
Git:            /opt/gitlab/embedded/bin/git

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