WIP Merge Request marked as Merged and sent merged notification

I opened a merge request and marked as WIP 3 weeks ago. Today, it fired an email saying that branch was merged without any interaction. I checked the branch and it is marked also as merged. When I checked the master branch, it didn't change.

I am using a GitLab Community Edition 11.2.3

Is this a bug? It is kinda alarming to us especially that branch is still in progress.

Here is the sample email that has been sent: