can't connect to raspberry pi UART TTL

I'm using the popular PL2303 to convert the USB signal from my computer to UART TTL signal by connecting it to the Raspberry Pi GPIO pins, following the guide here: https://cdn-learn.adafruit.com/downloads/pdf/adafruits-raspberry-pi-lesson-5-using-a-console-cable.pdf

I have done this with my mac, and it works fine, but I tried with 18.04 and 20.04, and it doesn't appear to send any output to the serial console, although it does seem to receive it for a while if you have screen listening to /dev/ttyUSB0 while rebooting the pi, but after a few seconds, it stops outputting anything.

It does work on Ubuntu 16.04, so I'm not sure exactly what changed there, but maybe that's a starting point.

I'm also kind of a noob with this stuff, so if anyone can tell me how to load the kernel module for 16.04 for the UART TTY stuff, that would be great as well.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.8.0-44-generic 5.8.0-44.50~20.04.1
ProcVersionSignature: Ubuntu 5.8.0-44.50~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-44-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Mar 2 20:10:03 2021
InstallationDate: Installed on 2020-08-19 (195 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: linux-signed-hwe-5.8
UpgradeStatus: No upgrade log present (probably fresh install)