Hi @DXM4, welcome to our forum!
I'm sorry to hear about that, sounds very strange. 
Do you have the STAMINA mode activated in your phone? This will be turned off when you start to charge your device.
Go to Settings > Battery > STAMINA mode > turn off, is it working now when you aren't charging the phone?
If there is no change, could you please try and restart your phone in safe mode and see if you experience the same behaviour?
Let me know how it goes! 
