Environment entries like GIT_COMMIT, GIT_BRANCH, etc. are null when using this integration.

pipeline {
agent any

stages {
stage('stage') {
steps {
echo "Value is: ${env.GIT_COMMIT}"
}
}
}
}

will output Value is: null

com.atlassian.bitbucket.jenkins.internal.scm.BitbucketSCM delegates a lot of calls to hudson.plugins.git.GitSCM, however method buildEnvironment(Run<?, ?> build, Map<String, String> env) is omitted and not overridden and the default implementation takes place causing all environment entries generated by Git plugin to be missing.