This will be added on a bit by others, but to summarize some information:
FLUSH SSL is not properly triggering the galera reset in our environment, and last night this caused our production DB clusters to fail.
Investigation shows that even in 10.5.10.7, FLUSH SSL is only causing the MariaDB port 3306 to pick up the new certificate, while WSREP on port 4567 keeps the old certificate.
Running 'SET GLOBAL wsrep_provider_options = 'socket.ssl_reload=1';' as the DB root user does cause WSREP on port 4567 to pick up the new certificate.
Looking at the test case added in commit c3b016efde4b1e0c2b85ca26c814ad43f5611ab2, I see that it only ever tests to see if reconnection is possible after running the SET GLOBAL, but while it does later run a FLUSH SSL, it then immediately goes into clean up instead of testing to see if that worked properly.
As such, I'm pretty sure that this needs to be reopened, and people trying to use this feature need to be aware that FLUSH SSL is still insufficient when using WSREP, and that a workaround is currently possible by adding the SET GLOBAL to the sequence.
Review and testing.