Getting strange subject in Update emails: "Predecessor task marked '$0' in $1"

Getting strange subject in Update emails: "Predecessor task marked '$0' in $1"

This just started recently.
I'm getting task update emails w ith this subject " Predecessor task marked '$0' in $1" .

The body of the email looks correct, indicating the closed tasks with a line through them and indicating the Successor tasks.

What could be causing this?
Thanks