Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hi Fiona,
Thank you for posting your question in this community. From your description, when you uploaded a Word document with Header to Teams and open it in Teams, the header doesn’t be displayed same as Word client does. May I confirm with you’re the header is display as following screenshots show?
Word Client
Teams
If so, when you open a Word document directly in Teams, it is open via Word for Web actually. According to this article about Differences between using a document in the browser and in Word. In Word for Web editing view, it doesn’t show page formatting which it includes page headers and footer. That is reason you open it in Word for web, the formatting is not shown as Word client does.
A lot of users also noticed this behavior in Word for web and post this idea on Word for the web UserVoice about Headers are no longer displayed in Word Online in Edit Mode. You can click hereto vote on this idea, if it is convenient. It can let the product team aware it and consider improve in the feature. Thank you for your understanding.
Because Word for web only let users make basic edits and formatting to the document, I would also recommend you edit the document in Word client. You can enjoy all good features in Word client compare to Word for Web.
Best regards,
Dihao