I am actually in one of those precious moments where the lapdock is working so I won’t unplug to see how exactly power delivery works with the charger 
However my first thought was that the battery was dying so I removed it and inserted the never-used one I had ordered with the phone a few years back, and that didn’t improve things. I’ve done the very-long reset once or twice as well.
One question about the things in /sys/class/power_supply, I see a bq25890-charger-0 and a tps6598x-source-psy-0-003f (and the battery). What is the difference between those? I think I remember seeing the charger enabled (“ONLINE”) and the “source-psy” disabled (ONLINE=0).