I would say that this is an EC bug, not a depthcharge bug. If the battery is too low and there is no AC power, the EC should have never allowed the system to boot in the first place. (If the AC gets unplugged while the system is booting and the battery is too low, the EC just has to force shutdown the AP... regardless of how far into the boot process the AP made it.)

There are two (separate) thresholds: one for EC and the other for Depthcharge. I guess EC's thresholds intended to be more permissive so that Depthcharge can show the battery low screen?

The range is pretty slim: the power supply has to be enough to boot the AP and show a firmware screen but not enough to run the OS.

Some boards have the same thresholds for AP and EC. So, the range doesn't exist and the necessity of this range is questionable.

Depthcharge is never showing a low battery screen. That's only done in the OS.

There's not point in having a different threshold for powering up and for continuing to boot once we're in depthcharge. They both lead to the same result (shutdown) if they're not met. If there's a difference there today, that's probably something we should fix. (Also, the whole point of doing this in depthcharge is to determine whether we have enough power to show a screen. We have not enabled the display yet at that point, and if we're below the threshold we'll shut down without every showing anything.)