Retrieving Custom XML Parts using OfficeJs in Word Online

I have a Word document that contains Custom XML Parts, and it works perfectly when opened in the Desktop version of Word. Using Office.js, I can successfully retrieve the XML parts with the following method:

Get CustomXML Parts by Namespace

However, when I open the same document in Word Online, the getByNamespaceAsync function returns an empty result. Worse, it appears that Word Online strips out the custom XML parts entirely when the document is opened.

Does the getByNamespaceAsync function not support Word Online? If that's the case, what alternatives should we use instead?

Thanks!