document part of the openvswitch issue that came up in team#40689

howto/upgrades/bullseye.md

@@ -537,8 +537,11 @@ See also the official list of [known issues](https://www.debian.org/releases/bul
 
 ## Pending
 
- * (from buster, needs confirmation:) upgrading restarts openvswitch
-   will mean all guests lose network
+ * the Open vSwitch upgrade completely broke the vswitches. This was
+   reported in [Debian bug 989720](https://bugs.debian.org/989720). The workaround is to use `auto`
+   instead of `allow-ovs` but this is explicitly warned against [in
+   the README.Debian file](https://sources.debian.org/src/openvswitch/2.15.0%2Bds1-2%2Bdeb11u1/debian/openvswitch-switch.README.Debian/#L236) because of a race condition. It's
+   unclear what the proper fix is at this point.
 
  * some config files should be updated in Puppet to reduce the diff
    with bullseye, see [issue tpo/tpa/team#40723](https://gitlab.torproject.org/tpo/tpa/team/-/issues/40723)