Here are some more details, the problem is not only seen with {app] :
It only fails to save the Substance scene, if we try to save a version that was already known to exist by the database.
For example, a task was saved with workfile v1. Then I change the template to add the {app}, restart everything (Ayon and DCC), and I try to save again (automatically version 1). This fails.
But, if I create a new task, everything works all right with {app}.
Then I remove the {app} from the template.
But I can still replicate the bug in another task, by doing the same trick : delete all workfile versions (on disk, not on database), and try to save a version from the Work File gui (which happens to be a version that existed before I deleted it on disk).
I guess that somewhere the database says “this version has already been saved”, and refuses to do so (which seems a healthy behaviour), but it should display a message to tell the user why the save was not allowed.