Battery is draining with "Power source: Power adapter" in macOS Catalina

I've noticed a strange problem with my 2018 MacBook Pro. Sometimes, when I use it with the (original) adapter, it says that the power source is power adapter, but the battery is still draining and not charging:

This is a huge problem as the level of charge can become very low before I even notice it. Also, I'm not doing any resource-intensive work. As an experiment I left it running and observed the charge get down to 40% before I disconnect the cable and reconnect so the charging would start.

What may be causing this issue?

it says that the power source is power adapter, but the battery is still draining and not charging:

There's three possibilities:

• Battery Management is Enabled. Catalina introduced a feature called Battery Health Management where it will lower the maximum charge while in this mode. If connected to a charger, it may appear that the Mac isn't charging.

• Defective battery. It happens. Even on new computers. Basically, the battery is not holding a charge meaning it discharges faster than it can be charged. In this case the battery needs to be replaced. There are some steps you can take to diagnose the battery.

• Defective charger. The fact that you said "Sometimes when you use it with the original adapter..." points to an intermittent problem. Your charger may not be able to supply the needed current to both charge the battery and power your laptop. There's also the possibility that you're using the incorrect charger. For example, you may be using the 61 watt charger when the MacBook Pro requires the 87 watt charger.

The easiest thing to do is to try a different and correct charger for your MacBook Pro. If the problem persists even after you try a different charger, it's likely your battery and you'll need to take it in for service.

I found that I could get a port that was not working to start working by unplugging the cable at the power adapter (leaving it plugged in at the laptop) and then reconnecting it. It's not clear to me what this is doing or if it's placebo, but it's odd.

Although my experience is with an older MagSafe 2 model, this may work for later MacBooks as well:

1. Look for debris / corrosion / gunk on both sides of the connection.
2. Plug charger out and shut down laptop
3. Clean both plug and socket terminals with toothpick / cotton swab immersed in pure alcohol or appropriate cleansing solution
4. Wait some seconds for solution to evaporate and reconnect

I too had this problem and then I found this article on Apple website https://support.apple.com/en-in/guide/mac-help/mh20876/mac . Read the third point on the website, try turning off the Battery Health Management in Energy Saver Preference. The charging resumed immediately on my Macbook after I did that.

Well, Apple care replaced the entire logic board but it did not fix the problem in my case. Later I discovered that is was faulty charging cable. Therefore, I suggest to check with alternate charging cable and charger first. This problem has nothing to do with macOS Catalina.