Most likely this is due to the smart battery health monitor they added to Catalina (with 10.15.5), it is too easily confused by a charge source that isn't the charger that shipped with the Mac.
Disconnect all displays & peripherals. Plug in the Apple charger that shipped with that MacBook into the left rear (closest to hinge) USB-C socket with the factory-shipped USB-C Charge Cable
- Go to Energy Saver, turn the battery health monitor off and restart the MacBook
- When it gets to the desktop, shut down
- Use the second option on this document (under 'If the issue persists') to reset the SMC (it forces a full reboot of the T2 chip): https://support.apple.com/HT201295
- Power up the MacBook and check that the charger is registering and that the battery is charging (if it is not, it is a hardware failure & requires service)
If you use a non-apple charger it will likely confuse the system again and you will have to do this procedure over.