Warning: Multiple merge bases detected. The list of commits displayed might be incomplete
I got the same message today. I just pulled the latest changes from the destination branch and merged them in the source branch and the issue was resolved.
Just take the latest changes of destination branch to source branch and complete the merge, after that this message will not come up on the pull request page.
In my opinion, this is good that Azure DevOps is giving such a warning so that if you forgot to take the latest from the target branch before finishing your pull request and it may save some time or end moment rush & surprise.