Edit (11th Aug 2020) : Reported bug to Abiword
https://bugzilla.abisource.com/show_bug.cgi?id=13934
Different font (here -> Nimbus Roman) is set as default when a user joins a public/collaborative Write Activity session.
Found on: Sugar 117-3, Debian 10.4 Buster
Reproduce: Join a collaborative Write activity session. A different font is seen only in the guest user, not in the shared activity's host.
Screenshot:

@Saumya-Mishra9129 reported Serif as the default in the guest user; packaged Sugar in Ubuntu 20.04
The font differs in different systems.