Actually, could it be that no droplet is working anymore?
Deleting and recreating the droplet doesn't work. It appears the droplet-code simply isn't registered in the output.
PS: I recreated this using WBPortable 2.8.3 SP5 Vanilla - nothing changed. Created a site with a ModifiedWhen-droplet. Everything worked. After an update to 2.8.3 SP6 it didn't anymore.