Hi Adam
I have just tested the Com Pi on a Pi 3B+ and it appears to be working correctly with the jumper attached. It booted and I was able to access the 1 wire and serial devices.
The GPIO connector pinout on the Pi 3 B and the 3B+ should be identical so I am not sure why it will not work on your Raspberry Pi 3B+ with the jumper attached, do you have it connected on the end of the Raspberry Pi nearest the SD card like in the photo below?
