Files on submissions remain in directory _sid_, are therefore could be cleaned by cron and are inaccessible to users

These files are attached to completed submissions as being in the _sid_ private file directory.

Indeed, they are and they are marked temporary. But what could have gone wrong that prevented them from becoming permanent?

(It looks like cron hasn't been running on an affected site. We'll fix that.)

They submitted the forms. That is, files attached to submitted forms are in "_sid_".

I sent it under a separate cover. The configuration allows drafts, and cron has not been running (although it is configured on the hosting platform to do so—that's a whole separate issue).

It turns cron on the affected site may be completing some tasks but not others because it has been dying with this exception.

Yes, the form is huge. I am down to it's either the half-broken cron executions or maybe, hugeness of the form.

Here's where I am:

1. It definitely happens for this one giant multipage form, although I don't understand under what conditions.
2. I can't yet reproduce it on a smaller representative form.

The affected files were uploaded in a file_managed that is inside a custom_composite, and they are all affected. However, I can't reproduce this by isolating this field in an otherwise empty form:

      other_institutions:
        '#type': custom_composite
        '#title': 'Other Institutions'
        '#title_display': invisible
        '#description_display': before
        '#multiple__min_items': '0'
        '#multiple__sorting': false
        '#multiple__operations': false
        '#multiple__add_more_input_label': Institution
        '#element':
          startdate:
            '#type': date
            '#title': 'Start Date'
            '#description': 'Approximate Start Date (yyyy-mm-dd format)'
          enddate:
            '#type': date
            '#title': 'End Date'
            '#description': 'Approximate End Date (yyyy-mm-dd format)'
          employer:
            '#type': textfield
            '#title': Institute/Employer
            '#placeholder': Institute/Employer
          funding:
            '#type': textfield
            '#title': Funding
            '#placeholder': Funding
          purpose:
            '#type': textfield
            '#title': Purpose
            '#placeholder': Purpose
          invitation:
            '#type': managed_file
            '#title': 'Invitation Letter'
            '#description': 'Upload copy of institution/employer''s invitation letter'

Whoops I didn't intend to change the status.

We are investigating https://docs.acquia.com/acquia-cloud/manage/files/broken/ as a possible cause.