Word 2609 bug sends SharePoint PDFs to a random folder

Microsoft confirmed a bug in Word for Windows version 2609: saving a document as PDF to SharePoint Online instead drops the file in INetCache\Content.MSO with a random filename, with no error shown. It only occurs when using Save/Save As with the PDF type and a SharePoint location; exporting or saving elsewhere works.

• Bug affects Word for Windows version 2609 when saving PDFs to SharePoint Online
• File lands in C:\Users\[username]\AppData\Local\Microsoft\Windows\INetCache\Content.MSO with a random name
• Word shows no error; workaround is Export or saving to another location
• Rolling back the update helps; Microsoft has not given a fix timeline