Hi!
Welcome to Microsoft Q&A forum.
What you are experiencing is a known limitation of the new web-based version of Outlook, specifically when a Gmail account is being used. The signature that you have set up in Settings > Account > Signatures will not automatically apply to a new or reply messages.
First, you can check the correct signature settings in new Outlook (Web)
- Open Outlook on the web.
- Click Settings → View all Outlook settings.
- Go to Mail → Compose and reply.
- Make sure:
- Your signature is pasted in the Email signature box.
- Check “Automatically include my signature on new messages I compose”.
- Check “Automatically include my signature on messages I forward or reply to”.
Reminder: In the new Outlook interface, Account → Signatures only stores the signature but doesn’t always show to new/reply emails.