It could be the charger or the battery. Batteries usually need a lot of amperage to charge, especially if you have downloaded it below 15%. It is important that your charger does not exceed the input voltage of your battery (normally 18.5-19V). But then you need enough amperage that it can swing. A low charger does not have the power to charge an empty or low battery or while the equipment is in use
Is there any means of powersaving installed? (TLP)? As you say it is charging fine when turned off or in sleep it tells me that the AC power does work, the system management chip works as well. Have you by any chance changed something around at charg thresholds for batterie to preserve it?
It’s been inconclusive. Sometimes it charges when I boot into Windows, but a few mins later I run into the same problem again. Even in Manjaro, if I leave it plugged in for really long, I’ll see that it has suddenly charged for a bit.
I’ve tried ‘resetting’ the battery (e.g. hold power button for 30 secs etc.), but that doesn’t seem to fix it either.
@pablomhr told you exactly that this could be happening because when the system is in use, it drains more power than the charger can deliver. This would explain the behavior. However, it is only a theory.
If you had a secondary charger for your laptop, you could test if this is true and the charger broke and is not as efficient as it should be.
Another test you could do is to boot into windows if you have dual-boot. If the same issue was there, it would mean a hardware failure. If everything was all right, you would know that something in Linux software is malfunctioning in your case.
When I had problems with battery, it was behaving the same way on any system, so I knew that my battery died.
Another theory could be that it’s not the charger or system’s fault but the battery is somehow broken and cannot be charged properly when in use. Why? No idea.
Anyway, if you don’t have dual boot with Windows, try some other distros live USB images, like Mint or Ubuntu to see if the battery issue is the same there. You can also try different kernels on Manjaro. With KDE software, you can check the battery state and so on. That is what I would do in your case.