Apt can't find package libasound2-dev 20.04 arm64

I'm trying to install libasound2-dev on a freshly installed Ubuntu 20.04 on my Raspberry Pi 4 but apt reports that it can't find the package.

Running apt search libasound2 yields:

ubuntu@ubuntu:~$ sudo apt search libasound2
Sorting... Done
Full Text Search... Done
alsa-topology-conf/now 1.2.2-1 all [installed,local]
  ALSA topology configuration files

libasound2/now 1.2.2-2.1 arm64 [installed,local]
  shared library for ALSA applications

libasound2-data/now 1.2.2-2.1 all [installed,local]
  Configuration files and profiles for ALSA drivers

ubuntu@ubuntu:~$

Looking at the package repository however, it should be available for arm64: https://packages.ubuntu.com/focal/libasound2-dev

My /etc/apt/sources.list is below (after removing commented lines).

deb http://ports.ubuntu.com/ubuntu-ports focal main restricted
deb http://ports.ubuntu.com/ubuntu-ports focal-updates main restricted
deb http://ports.ubuntu.com/ubuntu-ports focal universe
deb http://ports.ubuntu.com/ubuntu-ports focal-updates universe
deb http://ports.ubuntu.com/ubuntu-ports focal multiverse
deb http://ports.ubuntu.com/ubuntu-ports focal-updates multiverse
deb http://ports.ubuntu.com/ubuntu-ports focal-backports main restricted universe multiverse
deb http://ports.ubuntu.com/ubuntu-ports focal-security main restricted
deb http://ports.ubuntu.com/ubuntu-ports focal-security universe
deb http://ports.ubuntu.com/ubuntu-ports focal-security multiverse

If it helps, it seems like this problem isn't specific to libasound2-dev. It seems that apt can't find anything in the libdevel section.

I noticed that some of the files in /var/lib/apt/lists/ were zero size, so I ran:

sudo apt-get clean
sudo apt-get autoclean
sudo rm -rf /var/lib/apt/lists/
sudo apt update

Many of the zero size files were much larger and running apt search libasound2-dev yielded the expected result.