Is it a battery problem or SMC problem?

I have a mid-2015 MacBook Pro running High Sierra. Except when I'm traveling, I always use it in clamshell mode and connected to power. I rarely put it to sleep (I have Energy Saver set to just put the display to sleep).

On July 21 I noticed that the light on the power connector had gone to amber, but Energy Saver said that the battery was fully charged. However, coconutBattery said the battery was at 95%.

Today I noticed that the lightning bolt was no longer showing on the battery status in the menu bar, and when I click on it it says "Battery is not charging". Energy Saver says that the battery is at 97%, but coconutBattery says 81%. Here's its full report:

Does this look like a problem with the battery, power adapter, or SMC?

UPDATE:

I reset the SMC, and at first it seemed like the problem was solved. The battery item in the menu bar said it was charging, and eventually said it was fully charged, and the power adapter changed to green. At that time coconutBattery said:

But while I was posting an answer to the question, it changed to:

The adapter light is still green.

UPDATE 2

The adapter light changed to amber a couple of days after the above update, and the menu says "Battery is not charging". Energy Saver says that the battery is at 98%, but coconutBattery says 76%. Its history viewer shows:

Date                Health%     Charge Capacity Cycles
2021-08-01 17:21:28 80.29697    7030    8755    31
2021-08-02 09:10:45 80.28555    7029    8755    31  
2021-08-03 10:01:09 80.01142    7005    8755    31
2021-08-06 00:53:35 77.72701    6805    8755    31
2021-08-09 13:33:42 76.02513    6656    8755    31

And System Information says this in the Power tab:

Battery Information:

  Model Information:
      Serial Number:    C018104BAG5FWLMF1
      Manufacturer: DP
      Device Name:  bq20z451
      Pack Lot Code:    0
      PCB Lot Code: 0
      Firmware Version: 702
      Hardware Revision:    1
      Cell Revision:    3241

  Charge Information:
      Charge Remaining (mAh):   5959
      Fully Charged:    No
      Charging: No
      Full Charge Capacity (mAh):   6623
      Health Information:
      Cycle Count:  31
      Condition:    Normal
      Battery Installed:    Yes
      Amperage (mA):    0
      Voltage (mV): 12252

AC Charger Information:

  Connected:    Yes
  ID:   0x0100
  Wattage (W):  85
  Revision: 0x0000
  Family:   0x00ba
  Serial Number:    0x00f277bf
  Charging: No

So what should I look for to understand why the battery is draining instead of charging?

I tried putting the computer to sleep for a few hours yesterday, it didn't help.

You need to get the battery replaced. Even if it isn't flagging the 'Needs Servicing' flag, it's six years old, draining rapidly, with a diminished capacity.

80% of original capacity is Apple's threshold to replace the battery. If it's under warranty, they'll do it for free. If not (and yours isn't), you'll have to pay.

If the capacity is much higher, they'll tell you it's fine and to come back when it's lost a bit more!

It's around $200, but the good news is that because of the design, they actually replace the entire 'top half' - the keyboard, trackpad and upper case. So it's a pretty good investment to prolong the life of your Mac.