Enabling a uart dtoverlay on ubuntu server 20.04 for RPi4 stops it from booting

Hi all,
I recently upgraded my Raspberry Pi 4 from Ubuntu 18 to 20. In the boot/usrcfg.txt file, I have the following to enable one of the uarts: dtoverlay=uart3

Unfortunately this causes the pi to not boot at all. I have verified that when the offending line is removed, it boots fine. Furthermore I have tried using different values from uart1-uart5 - all result in the pi showing activity lights and then staying dead with no screen output at all.

I downgraded to ubuntu 18 and tried it again and it worked perfectly fine.

Thanks for your help!