I have a Data Table within one of my apps, this data table pulls data from a variable (varUserAdminList) which itself is fed on the OnVisible property of the app page as follows :
Set(varUserAdminList,Filter(AdminCollection, Active = true));
AdminCollection itself is a collection which is fed by a Sharepoint form called "Admin List". This Data Table, when setup, appeared to show values correctly. On reloading the app, however, it now shows as follows :

On checking the error I see the following :

The "Name" column is a person column, and Name.Displayname is DEFINITELY a thing for this data source.
Weirdly I also noticed if I go into the setup of the datatable and clear then reinput the "items" value of varUserAdminList it suddenly fixes itself :

If I then reload the app it again is broken until I change this value.