We use emojis in the Pull Request titles for marking different state/severity of the issue.
About a week or two ago emojis stopped displaying on Pull Request Summary page and instead show the code of that particular emoji. However, they still display properly when navigated to the pull request page.
Maybe it's somewhat related to https://bitbucket.org/site/master/issues/14417/title-of-pull-request-not-displaying-if ?
Summary page - No emojis
Pull request page - Emojis