I think the issue is that DefaultSelectedItems is returning a new record containing only Value, rather than the actual Person record from the SharePoint People column.
In your formula, you're doing:
{
Value: LookUp(
'Configuration List',
Title = "HR Admin",
User.DisplayName
)
}
This can make the person appear correctly in the ComboBox, but the record isn't necessarily the same type that the People column expects when the form is submitted.
I would try returning the actual User record instead, for example:
LookUp(
'Configuration List',
Title = "HR Admin"
).User
And similarly for Accounts Admin:
LookUp(
'Configuration List',
Title = "Accounts Admin"
).User
So the important part is that DefaultSelectedItems and the ComboBox Items should return compatible Person records, not just the person's display name.
If this is inside an Edit Form, also check the DataCard's Update property. Normally it should be something like:
ComboBox5_2.Selected
or, if multiple selection is enabled:
ComboBox5_2.SelectedItems
depending on whether the SharePoint People column is single or multi-select.
The fact that the default value is displayed but isn't saved strongly suggests a record-type mismatch between the ComboBox and the People column.
