Please check from the Forms server under the C:\ProgramData\Laserfiche Forms\RoutingError folder whether there are XML files under it(you can search for "XML" within the folder)  Forms has a recover mechanism that it will process the submission XML files under it when restart Routing Service. You need to identify which XML files are for your hidden form and delete them so they will not be processed any more.

To identify, first open the process diagram of the process and record the number in the URL which is the bp_id, then query from the Forms database " select process_id from [dbo].[cf_bp_processes] where bp_id={bp_id}" to find out the process_id, then check the value in "<ProcessId>4570</ProcessId>" from the XML files to see which XML files are for the hidden process.