Microsoft has confirmed that the September 2026 Windows security update KB5002914 breaks copy-paste, autofill and formula fill-drag in Excel 2024, 2021, 2019 and 2016. Pasted content does not appear at the destination and the source cell stays highlighted. No fix has been released yet. Microsoft advises uninstalling KB5002914 for now. If your office relies on Excel for quotes, invoices or stock sheets, check whether this patch has reached your PCs and hold off on installing it until a fix is out.