When the "i" icon of a reference field is clicked a popup view of the referenced record is displayed. However, the fields showing on this popup view is not the same ones configured on the sys_popup view of the form.
The sys_popup view of the referenced record displays as expected on Default view of the record but not on any other views (i.e. ess, Mobile).
Missing associated sys_popup,name_of_view.
The sys_popup view by itself only works for the OOB "Default" view.
Create a "sys_popup,name_of_view" for the referenced table (i.e. sys_user.do?sysparm_view=sys_popup,ess)
Product Documentation - Configure the reference icon view of fields
Community Blog Article - Configure a sys_popup view for a specific form view