Outlook has changed how it stores signatures more than once, and most companies run three versions of it at the same time. This page is about what that means in practice and how one template covers all of them.
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Signatures live in a folder in the user's profile as three files per signature, HTML, RTF and plain text. That is why a signature disappears when someone gets a new laptop, and why a copied signature loses its logo when the image was pasted rather than linked.
Newer builds store the signature with the mailbox instead of the computer, so it follows the person to their next machine. This is the behaviour a company wants, and the one a managed signature relies on.
Both read the signature from the mailbox. A signature set centrally appears in all three without anyone opening settings.
Outlook for Mac follows the mailbox as well. The mobile app keeps its own short signature, which is why a mobile variant exists as a separate part of the template set rather than a shrunken copy of the desktop one.
You design the signature once, map the directory fields, and deploy. AutoSignature writes to the mailbox, so classic Outlook with roaming signatures, the new Outlook, Outlook on the web and Outlook for Mac all present the same block, and the mobile variant covers the phone.
Why the first one happens, and what to do about it, is written out in why Outlook replaces employee signatures.
It is management software. Nothing is installed in Outlook, no add-in is deployed and no script runs on the endpoint. The signature is written to the mailbox from the tenant connection described on the Office 365 email signature management page.
They can, and the next rollout restores the managed version. If a team genuinely needs a variation, give it its own group and its own template set instead of leaving it to each person.
That case is covered on the Exchange email signature management page, including hybrid tenants where some mailboxes have not moved yet.