Auto merge issue with GitLab

Hi Guys,

I am writing this conversion to seek help on the problem faced by us in a critical project.

Problem: In our project, we have environment specific branches of our code repo e.g. QA, BETA, and Production. Now whenever a development of feature needs to be started, we always create a feature branch from production env and it always updated with production branch on daily basis.
As soon as the development of the feature is completed, we merge the feature branch to the respective env specific branches but we never update our feature branch from the target branches except for production branch.
Recently, we have faced an issue with the auto merge capability of git lab. We have taken an update from the production branch to our feature branch and git plays auto-merge on some of the changes where I was expecting a conflict. And in auto-merge activity, these changes of my feature has been removed.

Please help me to identify the root cause of the problem.