Outlook Add-in - AttachmentId is malformed - outlook

Question

We've been having issues intermittently where we get an error when downloading email item content from EWS "AttachmentId is malformed". This is for ItemAttachment (Especially .eml files)
We could not figure why or how this is happening and noticed that the ones that were failing had + and / in the id's. Searching across the web landed me on this article. Although this article is from 2015, wondering if this is still happening.
This article blew my mind and made sense (kind of) and implementing the conversion of + -> _ and / -> - worked fine, for a while.
We are now receiving the same error 'AttachmentId is malformed' and again could not find why, I removed the custom sanitizer function that replaces these characters and it started working again.
I have no idea what and why is this happening and how to reliably get attachment content. Currently, I've moved the sanitizer into the catch handler, so if for some reason the AttachmentId fails, we'll retry it by sanitizing it. Will have to keep an eye on how many fail.
Any light on this issue will be really appreciated.
Update 1.0 - Sample Code
Front-end
//At this point we've got the email and got the files
//We call EWS only if file.type == Office.MailboxEnums.AttachmentType.Item
//For all other files we call REST endpoint ~ Office.context.mailbox.restUrl + '/v2.0/'......
//Sample code below if only for EWS call
let files = this.email.attachments || [];
files.map(file => {
this._getEmailContent(file)
.then(res => {
return res;
});
})
//Get content from EWS
_getEmailContent(file, _failed){
//attachmentId
//Most of the times this will be fine, but at times when Id has a `+` or `/` if fails, Was expecting the convertToEwsId to handle any sanitization required.
let attachmentId = Office.context.mailbox.convertToEwsId(file.id, Office.MailboxEnums.RestVersion.v2_0);
return this.getToken(EWS)
.then(token => {
return this.http.post(`${endpoint}/downloadAttachment`,{
token: token,
url: Office.context.mailbox.ewsUrl,
id: attachmentId
},{
responseType: 'arraybuffer',
}).then(res => res.data);
}).catch(err => {
attachmentId = attachmentId.replace(/\+/g, "_");
this._getEmailContent(attachmentId, true);
})
}
Back-end
[HttpPost]
public DownloadAttachment(Request model){
var data = service.DownloadAttachment(model);
if(data == null)
{
return BadRequest("Error downloading content...");
}
return new HttpResponseMessage(HttpStatusCode.OK)
{
return data;
}
}
//Inside service
public byte[] DownloadAttachment(Request request){
var ser = new ExchangeService
{
Credentials: request.token,
Url = request.url
}
//Here it fails intermittently, returning AttachmentId is malformed.
var attachment = ser.GetAttachments(new [] {request.attachmentId}, null, null).First();
if (attachment is FileAttachment)
{
FileAttachment fileAttachment = attachment as FileAttachment;
fileAttachment.Load();
return fileAttachment.Content;
}
}