For the last week now, this has been happening to my colleagues and I intermittently.
We select 'Merge' The popup to merge comes up. Select to continue. Loader comes up for a bit Screen reloads and the Pull Request is classified as Declined But if you look at the the commit history, there is commit for the pull request merge.
This has happened to 3 of our users now and we have multiple examples of this in our deployment. Please let me know what additional info would be helpful to you.