I dont have any of those issues. Maybe is a manufacturing problem or something like that. Try the factory reset, I hope it works for you.
A factory reset didn't change anything. The problem of Gboard going away and being replaced by LG keyboard seems to be almost always a consequence of plugging the phone into the USB cable in my car, which activates Hands-free Link. And speaking of that, sometimes, but not always, when I unplug the phone from the car USB, the Hands-free notification stays active in the notification panel and cannot be swiped away. The phone has to be reset to get rid of it.
Under "Smart Settings" I have everything turned off. Nevertheless, when I plug in headphones, PowerAmp starts playing whatever it was last playing. There is no setting for this behavior in the app itself.
And as I already described, if I'm listening to a podcast on Podcast Addict, as I often am, if I unplug the headphones the podcast pauses (which is what it's supposed to do) and PowerAmp starts playing through the phone speakers (which it's not supposed to do) and I have to shut it off manually. This occurs even when I haven't used PowerAmp since the last phone reset.
Before we claim the bugs are in the apps, I'd like to point out that I had been using both Podcast Addict and PowerAmp on my OnePlus One (CM then LineageOS, Android 7.1) for three years without seeing these, or any other, issues. So I'm inclined to blame LG's version of Android, which is why I started this thread in the first place.
Another thing I've noticed since last posting: In the car, with the LG G6 connected via Hands-free, the car buttons to advance music to the next track don't work. They always worked before, with my OnePlus One.