So I had the same problem on my 12" iPad Pro 2018 edition being stuck on battery with red line after allowing it to go depleted and after the latest software update.
Tried multiple time with original charger and cable and an original apple MacBook Pro USBC charger to no avail, until....
I read somewhere else that some people had success when they used a different brand usb-c cable.
To test this theory I grabbed the other original apple usb-c cable I knew worked because it was charging my MacBook Pro and connected it and the iPad would still not charge.
So I grabbed a generic usb-c cable I had for an android device, connected it to my MacBook Pro port and plugged it in and out a few times from the MacBook Pro and low and behold it booted up and started charging at 1%. I then used the same generic cable and connected it to the apple charger and it kept charging and within 10 minutes reached 7% and kept charging...
At 14% I tried to switch back to either usb-c apple cable I had and it would not charge...
Another bug Apple needs to fix for when the battery goes dead and system refuses to recognize/charge with original apple cable.
This is a software issue folks...