* Update to clarify my above posts; I've resolved the Power Menu issues I was having on 14.1 to be related to Substratum Legacy System overlays. Uninstalling Substratum overlays on my device returns the full functionality of my reboot menu and Power button.
It does not, however, fix the long-press Back button to kill foreground app issue I mentioned. It still force-closes the System and performs a soft restart on my device, without overlays installed.
I've since went back to the I9195-LineageOS-15.1-experimental build by Arco, which by *far and large*, has simply the best battery life of any custom build I have used available here. The various missing features, in my case, can easily be overlooked, as most can be implemented via various 3rd party methods, and are definitely worth sacrificing in the meantime for such a battery life.
I've thought for months that my hardware has been on its last legs because of really over-excessive battery drain whilst using various different custom ROMs.
Resurrection Remix would be my OS of choice but I really can't justify using that fantastically feature-rich OS any longer on this handset, not after using the above LOS 15.1 build and remembering what it was like not to have to charge my battery every time I was near a power source. It's absolutely great!
* Sorry for the off-topic blurb, this was meant to be just a quick update about the overlays/Power Menu issue... :/