Power down
When I start my MacBook Pro while unplugged will shutdown even as the battery reads 20-35%, It quickly turns back on when I plug it in. But, I'm not sure why it's doing this or what the remedy is. Any thoughts.
MacBook Pro (2017 – 2020)
How old is your Mac exactly? From what you are writing, it seems that your battery has aged and cannot provide peak performance anymore. Here are more information about battery perfomance over time:
iPhone Battery and Performance - Apple Support
You can look up your battery health in settings to get info about how aged your battery actually is.
Maximum battery capacity measures the device battery capacity relative to when it was new. A battery will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges. Depending upon the length of time between when the iPhone was made and when it is activated, your battery capacity may show as slightly less than 100%.
A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge. Learn more about charge cycles.
To view your battery’s condition, choose Apple menu 
> System Settings, then click Battery 
in the sidebar. (You may need to scroll down.)
Read more here: Check the condition of your Mac laptop’s battery - Apple Support
