Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi,
You can try launching Classic Outlook; it should recognize the signature. You are likely using the "New Outlook" version from the Microsoft Store.
https://support.microsoft.com/en-us/outlook/install-or-reinstall-classic-outlook-on-a-windows-pc
Please consider rating the question (accepting the answer or not) and visiting the Microsoft Q&A community if you have further questions.
Thanks.