Update: acpi=off only got me to a timestamped kernel message "reboot:System halted". acpi=force had no effect. Repeating report to Bug 1776616, which is specifically relevant to Kernel 4.15-n-generic on xubuntu™ 18.04-LTS.
Update: acpi=off only got me to a timestamped kernel message "reboot:System halted". acpi=force had no effect. Repeating report to Bug 1776616, which is specifically relevant to Kernel 4.15-n-generic on xubuntu™ 18.04-LTS.