@Margo I had the user replicate the issue. They're opening the SharePoint form in full screen, and are uploading 2 document simultaneously - an email (.eml) with a pipe character in the name, and a .pdf without special characters:
Documents uploaded without failing validation:
It doesn't throw any error after adding these but when he saves, it does not close the form, but submits the SharePoint record anyhow.
After he removes the offending file, he is able to save, which creates a duplicate SP item:
To troubleshoot, we had him just upload the failing .eml file, and he was still able to upload it without error but showed the same behavior listed above - it errored the form but still saved a new SharePoint item.
Hope this helps - let me know how else I can help!