NetworkManager tries to enable an interface only once

Issue

After a network outage, NetworkManager-managed 6.2 machines lost their IP addresses and required manual operator intervention to restart the network.

This is surprising as 4.x and 5.x machines (before the NetworkManager) did not lose their IP addresses during similar network outages and continued to operate normally after network connectivity was restored.

This is the network diagram:

MYHOST---SWITCH1----SWITCH2---DHCPSERVER

This is what happened:
- power was lost on SWITCH1 and SWITCH2
- NetworkManager detected link loss and disabled the network on MYHOST (as expected)
- power was restored on SWITCH1
- NetworkManager detected link presence, tried to acquire an IP address (with a 45 sec timeout) (as expected)
- since SWITCH2 was still down, there was no DHCP reply
- after the 45 sec timeout, NetworkManager disabled the interface, there was no further NetworkManager activity afterwards (wrong - expected the NetworkManager to try several times, if not forever, before giving up)
- power was restored to SWITCH2, but the NetworkManager on MYHOST cannot see this