Workflow task layout wiped out by Designer

See the attached video (plus audio).

See targuillere-SANDBOX, branch NXS-5916.

Basically:

• A simple workflow, one task. One WF variable, docs, type document, multivalued
• Must be filled by an operation (not the default nuxeo-document-suggestion page provider, not even a custom page provider: An operation)
• The operation accepts a parameter, here folderId
• (make the WF variable docs available in the task)
• in Designer, scaffold the task.
• Setup values: the operation to use, and set the parameters to ` {"folderId": "[[document.id]]"}

  `

• When saving a strange error about dublincore is displayed
• Then, when you re-save, the layout is wiped out and becomes totally empty.

It is not 100% reproductible. My first try to reproduce failed (no problem, no removal of the layout's content), the second worked.

Suspicion goes to the fact that parameters are added, of course. Maybe there is a strict way to enter such values (if yes, displaying a tool tip would be nice)?