Yes - I read your comments.
Also the one where you prefer fixes to workarounds - Linux is all about workarounds 
If you check the Branch compare for Manjaro web and search for webkit2 you will find that all related packages was updated the same day.
Another topic referring to back-in-time issue on Arch forum and eventually the Arch bugtracker
The webkit greeter was a project of Antergos and the project is not developed anymore (archived)
Comment on Arch bugtracker with reference to the webkit-greeter
Edit: I think project is abandonned, repository is readonly, I will switch to lightdm-gtk-greeter, this one works
– FS#70099 : [gobject-introspection] lightdm-webkit2-greeter fails to run now since gobject-introspection update
This brings me back to my statement on theme maintenance as the greeter is depending on the Gnome theme API which has changed and is a likely cause for the greeter to fail.