Buprenorphine also increases cardiac mortality by increasing the QT interval. Buprenorphine is used over methadone mostly because its a partial agonist of the mu-opioid receptor which basically limits how high you can get.
You’re right! I hadn’t known that buprenorphine also caused it, but it seems all of the drugs used for that purpose. I did find a 2013 study stating that the effect was significantly less than that from methadone, but it’s good to know that both come with cardiac risks.