I never adopt a new edition of macOS until it's 2nd or 3rd major revision so I haven't installed Tahoe but I've seen a number of reported UI issues arriving with Tahoe & certain Office updates.
Make sure you have Tahoe fully updated [26.0.1] & Office updadated to 16.102.2 [Current Channel (Preview)].
If the issue persists in more than one workbook use Help>Feedback in the Excel menu to report the problem to the development group.