Hi @thedmnd1
May I ask a few questions please.
1. Are you using a Samsung Charging Brick and cable when charging up the phone ? Ensure the cable is free from damage as well as the usb-c connector.
Perhaps try another.
2. Is the phone turned On when charging up the phone ? If so it sounds like your phone us consuming more power than what is being put into it.
Try > Safe Mode > https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/ turns off downloaded apps and themes etc which may highlight an app or theme that's causing conflict.
Personally I'm using a Samsung Charging Brick and cable and usually at night have my phone powered down and it charges fine.
Although if I need to ever top up the battery through the day I use a Samsung Wireless Charging Stand, with the phone turned On and what's happening with your phone does not happen with mine.
Try a Soft reboot by turning the phone Off and On.
Ensure your phone's software is updated.
Make sure your usb-c port is clean of debris too.
Please also try the following >
Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
There is a diagnostic tool in your Samsung Members App.
If this continues then as a last resort back up and factory reset.
A Samsung Service Centre can check out the phone.
I wish you all the best. 
