Missing host-initiated shutdown command
Usually after the UPS sends the host the "shutdown now" message, the host sends the UPS a message that means “I acknowledge your shutdown request, I have finished preparing for power down, and now require the UPS to turn the host off for a minimum of 20 (or some configured value) seconds.” The state diagram on A&D has only one of those messages, but the UPSes I use (going all the way back to strangely-wired RS232 signal lines) have two messages in this protocol.
After commanding the UPS to shut down, the host will typically put itself into some state that only a power cycle can get it out of (e.g. it goes into S5 state), and further communication between host and UPS will not be available. So that 20 seconds of off time is a minimum to be observed in all circumstances, even when AC power is available. Sometimes this protocol is used as a “host reboot” feature, since it also achieves a reboot of connected devices when AC power is on.
If AC power is still unavailable at the end of the 20 second delay, the host should not be turned on again until AC power (and optionally UPS charge) is restored. The host->UPS shutdown message is the host’s way of saying “I’m out of here, let’s try again when power is back to normal.”
I think you are expecting the host will change its load current to provide this signal to the UPS. I’ve seen that idea fail enough times by now to stop bothering to try. Current sensors will either turn off an energy-efficient host too early, or fail to turn an earthburning host off at all. The explicit USB signalling, when available, is much more reliable.
This also avoids having to have some way for the UPS to abort a shutdown (i.e. on transition from "User warned of shutdown" to "Poll of mains reports good AC voltage). If the host sent a shutdown command to the UPS, the UPS must observe it; if the host sent no such command, the UPS is free to keep the host powered and go back to the "Charge wait" state.