matrixsynapse: Matrix Synapse not installable on stable images with backported freedombox package

I built a new stable image of FreedomBox with the freedombox package installed from buster-backports.

I noticed that Matrix Synapse is not installable in this setup unless this command is run first
sudo apt -t buster-backports install matrix-synapse

Also reported on the forum:
https://discuss.freedombox.org/t/solved-error-message-installing-matrix-synapse-app-on-freedombox-raspberry-pi3b-rasbian-buster/798