What is the logic of the new version of gitlab merge

Currently we are using an old version of gitlab. When MergeRequest, the diff of the base commitId and target Commit of two different branches is displayed. Our gitlab has dealt with a problem. The two branches have the same base, and the same content is submitted respectively. When the two branches are merged, The gitlab service does not compare with the Base commitId, but thinks that there is no file change. How is this done?