Can you please provide us a sample expression to be used in the "default filter" section of a service-backed record?

Which version of Appian are you using? The required expressions for Expression / Service backed records vary somewhat depending on the Appian version.

7.7

According to Appian Documentation: To add default filters to a service-backed record type, enter an expression using the a!queryFilter function in the Default Filters field. Appian recommends defining the default filters expression as a rule for version control and testing purposes and calling that rule in the Default Filters field. - forum.appian.com/.../Record_Design.html

jackm beat me to it - though just in case here is the 7.7 version of the documentation (it looks basically the same): forum.appian.com/.../Record_Design.html