how to remove/deal with locks in a MariaDB Galera cluster
The only way that I found to fix this issue is to restart the whole cluster in the original order as restarting a single node just fails. I'm not sure why this is a thing in Galera, it seems to be very sensitive to errors like this. In my opinion a database cluster should just work no matter what queries you throw at it.

I guess it's something of a free add-on when you go the Galera road, unfortunately.