Why can Copilot for Word no longer read locally saved documents when uploaded as a source?

Prior to the most recent update, on or about 7/15/26, locally saved documents could be uploaded into Copilot in Word, and Copilot could then read them in the background as source material. Since the update, Copilot reads every upload as missing a header and states that the upload is a blank document. I was using this feature extensively and successfully prior to the update, so what changed?