Possible pull request merging bug (Mercurial)

I've encountered an issue with merging a pull request. I'm using a Mercurial repository.

A pull request was created for branch x and was to be merged into branch y (destination branch). After creating the PR branch y was merged (into default) and closed. I did not know that and clicked on "Merge" button on the PR's page. After I accepted the merge message the loading icon started spinning and was spinning, and spinning... After about 30 seconds I refreshed the page and did the same thing – to the same effect. It turned out that:

• The x branch has been closed, but not merged.
• The PR was still in "open" state. When I clicked on "Edit" button to change destination branch the edit page only loaded partially: there was spinning loading icon under "Update pull request" button and list of commits didn't load.

Finally, I declined PR and merged the x branch manually to the correct branch (default). It worked correctly, as expected.