Update. I had zram-tools installed as well. Or at least, I have it after the upgrade to 21.04
I removed it and zram-config, and reinstalled zram-config, and it works fine.
However, this still doesn't make much sense: if they were both installed previously, why did it stop working?
I stepped through the /etc/bin/init-zram-swapping and works perfectly well.
Update. I had zram-tools installed as well. Or at least, I have it after the upgrade to 21.04
I removed it and zram-config, and reinstalled zram-config, and it works fine.
However, this still doesn't make much sense: if they were both installed previously, why did it stop working?
I stepped through the /etc/bin/ init-zram- swapping and works perfectly well.