You're working with both Outlook (desktop) and Outlook on the Web (browser) and therefore want to use the same default signature in both clients.
If your Outlook on the web (OWA) email signature is incomplete, you have reached the character limit for OWA.
Microsoft enforces a 8KB character limit in iOWA email signature settings.
The maximum signature size in OWA with Exchange Online (Office 365) is 8KB for HTML and text signatures.
- Clean your HTML code and remove redundant (invisible) HTML tags.
- Remove line breaks which account for 2 characters each.
No worries, you can still have a great looking email signature within the 8KB size limit.