Just checking to make sure you performed the SMC correctly since your laptop is a T2 model where the SMC Reset procedure has changed from earlier Intel Macs. I personally like to Option Boot the Mac and let it sit at the Apple boot picker menu when I perform an SMC Reset so that I know that the SMC Reset has actually been activated because it will cause the laptop to immediately power off.
Here is the Apple article with instructions for performing the SMC Reset for various model Macs including the T2 models (read the instructions very carefully & perform it exactly as written):
Reset the SMC of your Mac - Apple Support
Also, I have seen a lot of the 2016+ USB-C Apple laptops require the power adapter to be connected even when the battery has a sufficient charge. Generally this is rare, but I have seen it with a lot of laptops I have serviced. I believe the laptop's Logic Board loses contact with the Battery for a moment after which these USB-C Apple laptops tend to need the power adapter to be connected in order to power on. I had a brand new battery have this problem, so I suspect the problem is more with the Logic Board than the Battery, but it is hard to say since I've never investigated it enough to figure out the source of the problem.
With a 2019 model.....if this is the original battery from the factory, then it may very well be worn out or even failing....it is difficult to determine this without special testing of the battery beyond what the Apple Diagnostics provide. Or it may be an issue with the Logic Board or a combination of both.
You can try disconnecting all external devices from the laptop just in case one of them may be causing a problem.
You can try running the Apple Diagnostics to see if any hardware issues are detected. The only useful diagnostic is one which reports a problem. Even if the diagnostic reports a bad battery, it may not necessarily be the cause of your specific issue here. Even with my years of experience, I cannot say it a new Battery would solve your specific issue with this laptop.
Personally I don't recommend spending any money on repairs for any Intel Macs for multiple reasons. The money spent on repairs is better put towards a new laptop.
