I used to be an Android device developer back in the Lollipop days. I compiled the various images myself, including the bootloader, recovery, and the Android OS. I can say with 100% certainty that at least back in the Lollipop days, and at least on the vast, vast majority of devices (a device could theoretically change this, and I don't know everything about every device ever released), the battery percentage that shows up when the phone is off is part of the bootloader, not Android. It's a separate image entirely.
I've rooted many android phones including modern ones. Flashing the bootloader/recovery without the OS always changed the charging animation in the off state in my experience. Can't say this is how all Androids work, but I've never encountered one that didn't behave this way.
I just know that when I flash anything Linux the offline charging indicator stops working
You most likely are flashing the bootloader, recovery, and OS all in one step. They can be combined into one image and all flashed at once. I doubt the Android bootloader would be able to boot Linux, but tbh, that's not my area of expertise, so I could be wrong.
I used to be an Android device developer back in the Lollipop days. I compiled the various images myself, including the bootloader, recovery, and the Android OS. I can say with 100% certainty that at least back in the Lollipop days, and at least on the vast, vast majority of devices (a device could theoretically change this, and I don't know everything about every device ever released), the battery percentage that shows up when the phone is off is part of the bootloader, not Android. It's a separate image entirely.
I've rooted many android phones including modern ones. Flashing the bootloader/recovery without the OS always changed the charging animation in the off state in my experience. Can't say this is how all Androids work, but I've never encountered one that didn't behave this way.
I just know that when I flash anything Linux the offline charging indicator stops working
You most likely are flashing the bootloader, recovery, and OS all in one step. They can be combined into one image and all flashed at once. I doubt the Android bootloader would be able to boot Linux, but tbh, that's not my area of expertise, so I could be wrong.