When a SQL KILL statement is requesting a transaction to be aborted at the same time when the same transaction is chosen as a victim in the Galera transaction certification process, the server can hang.
There have been attempts to fix this problem earlier. A suggested fix for
MDEV-18464 had been pushed and soon thereafter reverted because of issues. Another fix (which adds another field to THD, expanding the potential state space) was pushed to 10.4 and 10.5 in MDEV-21910, but it fails to prevent such hangs.
It seems possible that something related to this caused
MDEV-17092, which I had worked around by changing the code in InnoDB.