Unable to delete form submissions from code

We have a scheduled job almost identical to the one detailed here : http://www.herlitz.nu/2018/10/15/episerver-forms-cleanup-for-gdpr/ for deleting user data when that database is used in a non-production environment. When this runs there is an NullReferenceException exception thrown on 3/70 forms when its attempting to delete the submissions

Object reference not set to an instance of an object
at EPiServer.Forms.Core.Data.FormDataRepository.DeleteUploadFile(ISubmissionStorage submissionStorage, FormIdentity formIden, String submissionId)
at EPiServer.Forms.Core.Data.FormDataRepository.DeleteSubmissionData(ISubmissionStorage submissionStorage, FormIdentity formIden, String[] submissionIds)
at EPiServer.Forms.Core.Data.FormDataRepository.DeleteSubmissionData(FormIdentity formIden, String[] submissionIds)

I then tried clearing one submission at a time on the problem forms

foreach (var sub in submittedData)
{
	sub.Data.Clear();
	submissionStorage.SaveToStorage(formIdentity, sub);
}

which revealed that there was one submission each form that had an issue and the exception is now a FormatException

Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).
at System.Guid.GuidResult.SetFailure(ParseFailureKind failure, String failureMessageID, Object failureMessageFormatArgument, String failureArgumentName, Exception innerException)
at System.Guid.TryParseGuidWithDashes(String guidString, GuidResult& result)
at System.Guid.TryParseGuid(String g, GuidStyles flags, GuidResult& result)
at System.Guid.Parse(String input)
at EPiServer.Forms.Core.Data.DdsPermanentStorage.SaveToStorage(FormIdentity formIden, Submission submission)

If I create a form submission myself I can see that the id of the submission was 8658704:CFA0CC6C-2973-493E-A3A8-B6C05DC07DA8 and there is no exception when it is cleared by the job.

But for one of the submissions that is throwing the exception the submission id is 7883982:5D8D3B79-D542-4543-BF59-21786095E7F5 which looks valid but still throws that exception.

My plan was to alert us to the exception with information about the form and the submission id so that we could go to the Production CMS, find that submission and delete it so that it wouldn't bother us again. I've discovered though, if I select that single submission in the CMS and click Delete, then I get the NullReferenceException again and the submission isn't deleted. 

System.NullReferenceException: Object reference not set to an instance of an object.
   at EPiServer.Forms.Core.Data.FormDataRepository.DeleteUploadFile(ISubmissionStorage submissionStorage, FormIdentity formIden, String submissionId)
   at EPiServer.Forms.Core.Data.FormDataRepository.DeleteSubmissionData(ISubmissionStorage submissionStorage, FormIdentity formIden, String[] submissionIds)
   at EPiServer.Forms.EditView.Controllers.FormsDataStore.DeleteData(ContentReference contentLink, String[] submissionIds)
   at lambda_method(Closure , ControllerBase , Object[] )
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(Controller

However, if I select all entries on the form  and then click the Delete icon then it does clear all the form submissions without any issues. Deleting all the submissions in Production isn't something that we can do or the client wouldn't be very happy!

I'm wondering if anyone could

a) Suggest what might be wrong with the form submission and a way to fix or delete it. The stack traces suggest it might be something to do with the uploaded file on the submission, but looking at them in the CMS there is no file upload shown on the submission - perhaps it has a partial record of an upload?

b) Help me understand how the submissions are deleted en masse from the CMS Delete icon when all form items are selected so that I can use this different approach in the scheduled job.

For some background, the site has recently been upgraded from EPiServer 9.10 to EpiServer 11.11 and its hosted in DXC. However, all the problem submissions are prior to the upgrade.

Thanks

Ed