I try to calibrate my battery every once in a while https://www.lifewire.com/how-to-calibrate-android-battery-4777614 and I find that when I plug my phone in after it drains to zero and dies, grapheneos attempts to boot immediately, then once it boots it shuts down to low power, then begins to boot immediately, then shuts down due to low power, and cycles through this several times before it finally builds up enough charge to stay on after booting.
I believe this is caused by it automatically booting when it's turned off and power is connected, however, this leads to this issue when the battery is too low to handle the power draw used by the boot process. I think Grapheneos should check the battery before booting when plugged in, to make sure it has enough that it won't need to shutdown immediately after booting