Only a vague one. You seem to know systemd, so maybe you can make more sense of the design:
Installing Duplicati on Linux (Arch / Manjaro) looks like their packagers set it up with a user unit file.
systemd/User from Arch Linux says where it might be, if you want to edit (or I think status finds it).
/etc/default/duplicati is another way to add --no-hosted-server option so TrayIcon only does that.
A question to me is whether Server will complain, but it might not matter. It probably ignores extras.
Duplicati.GUI.TrayIcon.exe is the place where the option fits. Note it has trouble with GUI password.
If you want more help, the “Installing” link above should help reach some people who know Manjaro.