[Nexus 6P Android Development] AzraelsKiss:
A

NEW

clean install... Not one of the two installs that you performed prior to my suggestion that you are having your issue on... A new clean install... A brand, spankin' new one, that you haven't corrupted with themes and who knows what apps that may have caused problems with your SystemUI... A not old clean install that is new! And in case you are still not comprehending how you did not do a clean install after I suggested it, here's a diagram...

Clean install + vendor.img + Ti restore + theme + apps > SystemUI crashed at 15% > Clean install + theme + apps > SystemUI crashed at 15% > Removed theme > SystemUI didn't crash at 15% > SystemUI crashed at 15% > Asked for help > Obtained suggestion for new clean install > Claimed new clean install, but issue remains...

Let me explain your issue for you and perhaps you might understand why I recommended what I did, instead of just ignoring it...

The ROM, in and of itself, does not crash the SystemUI at 15% on its own. I have ran the battery on my device below 15% numerous times, and even down to 0% a couple of times since Nate released PN7 and not once did I get a SystemUI crash. So therefore, it would seem as though it is not an issue with the ROM. If it's not an issue with the ROM, then it is an issue with your device. This could mean one of two things... A hardware issue that cannot be fixed within the parameters of this forum (but is also fairly unlikely, because a hardware glitch wouldn't likely manifest only when the battery system in the OS is showing precisely 15% remaining), or a software issue (way more likely and something that may be diagnosed and even rectified within the confines of said forum). So lets assume it's a software issue... So what sorta software would affect the SystemUI at precisely 15% battery remaining??? Hmmm... Probably any sort of software designed to make changes to the system once the battery starts getting low, and is programmed to make these changes at a specific trigger amount of... Let's just say for the sake of argument... 15%. Bad news is... According your account... You have absolutely, positively no battery apps that could do this. But... What if I didn't believe you? What could be causing the battery saver, that you're not using and absolutely don't have, to misbehave when it's activated? Let's, for just a moment, consider what happens when the batter saver is activated... It disables a lot of battery consuming activities and change the UI colors for the status bar and navbar. Hmmm... I wonder what may happen if something were to get in the way of changing those UI colors... And what could possibly affect the system's attempt to change the UI colors when battery saver is activated??? Perhaps something that normally changes UI colors, but perhaps isn't properly updated for N or setup correctly, so therefore it interferes with the change... Maybe something like themes... But the themes were removed... How could that still be getting in the way? Well... If I do a really, really, really simple search about substratum (and similar theme engines such as layers and rro) I find that removing the theme may not remove all the changes made by that theme... So how do we remove the rest of those changes? Whadya know... The same search explains that these changes are likely in /system/vendor/overlay, so deleting that directory, or reinstalling the vendor.img, and rebooting could correct that issue... Weird... But for the sake of argument... What if you are completely right and absolutely nothing is affecting battery saver (even though you claimed that battery saver won't even activate when you try to manually activate it)... What other software could be crashing the SystemUI at exactly 15%? Hmmm... Could it be an app causing issues? Or mayabe a corrupted install? Or something that just needs to be cleared out? Perhaps... But what's an easy way to take care of all these things in one fell swoop? I'll give you a hint... It starts with new... Ends in install... And might be clean in the middle...

Oh well... Now that I've wasted plenty enough time and space on this issue... Do what you will. Good luck.

Have a good one!