Opening this as it was discussed already in slack.
When the upgrade is done PXC nodes are upgraded 1 by one (writer last), but when one node is terminated and becomes online it is not added immediately to proxysql config so you can get into situation without online hosts in reader group.

Here's how it looks:

node-2 was upgraded and ready, but it was not yet added to proxysql config and node-1 was then terminated for upgrade (SHUNNED status here), so no nodes left for reads