Mariadb 10.1 / Galera - bootstrap node always doesn't lose quorum
I don't think this is a bug. Its an expected behaviour interms of normal shutdown scenario. If you are practicing split-brain testcases then it makes sense for it to loose its Primary status.

In your case the clear information about their neighbouring voters exit status is communicated. And thus software lets it to serve the requests to the node1 that is alive, since the status of other nodes are shutdown and that is communicated to node1.

Try to execute Split-Brain scenario by isolating the node1 from node2+arbitor link and then you should find node1 losing its primary status and node2 wins while arbiter voting by its side.