I’m reasonably certain I’ve found the problem, and im working on pushing a new package out with a fix now.
In the mean time, after installing sympl normally, manually this to enable the missing part of the configuration:
sudo a2enconf sympl-web
sudo service apache2 stop
sudo service apache2 start
It’ll take a while for the potential fixes to come down the pipeline with new packages, and I will also need to re-replicate the issue to confirm before they’re pushed to stable, but this should deal with it for now.