Undefined array key "#webform_key" in WebformManagedFileBase::validateManagedFile()

Problem/Motivation

This error occurs for obscure reasons after the 9.5 Drupal core update and prevents uploading files (thus breaking the form).

This issue does not occur on Drupal 9.4.9.

The only clue provided by the logs is a warning:

Undefined array key "#webform_key" in WebformManagedFileBase::validateManagedFile() on line 869 of WebformManagedFileBase.php

Using xdebug, I've been able to track down the issue to the custom composite's file element, which indeed is missing a "#webform_key" value. Removing the custom composite allows uploading files again.

Steps to reproduce

Install Drupal core 9.5.3 and the webform module; then simply add the attached form and try submitting a file (on any field).

The upload should fail. Clicking on the submit button will display a message warning the user that they might lose data because the upload is still under way.