I had the same symptoms on my phone, wouldn’t go past 50%, wouldn’t really show it’s discharging. I would reset it a million times, and sometimes while powered up I would get in and it would go back to normal. Oddly enough I noticed that when i would set my phone down, it would revert back to the 50% and the LED charging light would turn off again. So… I wondered, hmm maybe it is the USB port, i forced the charger pin inside the port down a bit, hooked up the cable and pull it down a bit (my cable isn’t in top notch shape but it works). AAAnnnndddd VOILA,,,, charging LED turned on, and the battery went back to normal and it would say charging again, fiddling with the cable a bit or pulling it in the wrong direction and boom the light would turn off and we’re back to zero. So, in my case, it is a damaged port, I mean the phone is well over 5 years old now and it’s still going.
My solution, really pull down on the pin and it’s been working for a day now again. I ordered a replacement usb port and a battery just in case, but those things take time to get to me (1-2 months). For now it’s fine, I do have a oneplus 7 pro i bought recently as my main phone now, but I’m determined to restore my OPO back to normal with those fixes.