If Webform does not validate in submit the uploaded files in dropzone are lost

Actually this happens when the DOM is refreshed so AJAX request (multi-step forms) etc. will clear the DropzoneJS widget of the files. Files are still submitted with the form.

I'm having this issue too, using Webform Cards.
Dropzone required attribute is not handled with Clientside validation, it doesn't prevent from going to the Preview stage, validation fails, I'm redirected to a custom Webform Cards page with all validation errors.
There, all files are missing, and if I upload the missing file, every file is in required error (even the one I just uploaded, which can never be validated)