Greetings @To
Thank you for providing the workflow.
I can confirm that indeed, if the attached document is a .docx the exported attachment does come with blanks replaced by + symbols. I am not sure what may be causing this but I do not think it is something that can be configured in ArcGIS Online.
I did a test where I saved the test .docx as a .pdf and that did not have the same effect. However, the behaviour of what happens to the file is not the same either because when you click on the PDF attachment, the document is opened in the browser where you can save the document and have the option to replace the "+" with "_". I know this is a quirky workaround and not an ideal solution.
I would suggest raising the issue with your local Esri Technical Support team so that they can investigate the issue. I will continue to investigate on my side and will post if I find out anything useful.
I hope the above is of some help.
Miguel