After installing a new signature to the email client, the signature may sometimes revert to what you have used before.
The previous signature provider can access your Google Account settings and force the changes. This is exactly what happens when you see the signature reverting to the old one.
That is the case if your signature is being reverted to the one with a branding line that looks like this:
To prevent this from happening, contact the previous signature provider and request to delete your Google credentials from their system, delete your account and all software provided to you by the company (extensions, add-ons, applications, etc.)