We have a community which has a lightning:fileUpload component for guests to upload anonymous file attachments to their created records. The guest user creates a new record via a global action which then takes them to a modified record detail page containing the file upload component. We accomplish this by enabling the guest user file upload checkbox and providing the record id to the file upload component.
As of this morning, Salesforce have rolled out the 'Assign Records Created by Guest Users to a Default Owner' security update, which, when enabled, blocks guest users from uploading files, even if they have the modify all permission on the custom object.
Is this a Salesforce bug which needs a case raising or intended functionality of this update as there is no mention on the update documentation (https://releasenotes.docs.salesforce.com/en-us/winter20/release-notes/rn_networks_guest_user.htm) and the enable guest file uploads still remains visible and checked. If the update is disabled, the file upload component immediately works again.