Battery shutdown below 50% needs connecting to mains when try to reopen

This has been a common issue with the Retina model laptops for some years now, but it especially got worse with the USB-C Macs and doesn't have anything to do with the charge level of the battery. My guess from observing Apple USB-C laptops during my repairs is that the Logic Board thinks the battery was disconnected since the power adapter is usually required to power on these laptops the first time after the battery is reconnected. I've seen this occur even with a brand new Apple OEM battery so it is not a battery issue unless the interface board on the battery is at fault....I suspect a Logic Board hardware or firmware issue is the cause of this issue when the battery is still healthy.

However, if your laptop has a failing battery, then that could be the cause as well. What is the battery condition as reported by macOS? If it is showing "Service Recommended" and you are experiencing battery related problems, then most likely the battery needs to be replaced. You can also try running the Apple Diagnostics to see if any hardware issues are detected. A passing diagnostic does not mean there are no problems. Unfortunately it is very hard to identify failing batteries if these steps do not show a problem.

If you have an Intel Mac, then you can try an SMC Reset.

FYI, Lithium Batteries start wearing out & failing between 3 - 5 years although there are exceptions.