'kill <thread id>' gives ERROR 1095 (HY000): You are not owner of thread ... even though I am

I did an ALTER TABLE on a huge table on a MariaDB Galera Cluster 10.2, but after a while of waiting I changed my mind and tried to kill the connection. However, MariaDB said ERROR 1095 (HY000): You are not owner of thread 123 even though I most definitely am the owner. So what is going on?