Hi @sposvc ,
Which type column do you use to store the cboExecMgr selected value within your Main SP list? A Person type column or a Single text type column?
Based on the needs that you mentioned, I assume that you also use a Person column in your Main SP List to store the cboExecMgr selected value, is it true?
I think there is something wrong with the formula you typed within the Update property of the ExecMgr Data card in your Edit form. I have made a test on my side, please take a try with the following workaround:

Firstly, you need to add a Office 365 Users connection within your app. Then set the Items property of the ComboBox (DataCardValue15) within the PersonColumn Data card in the Edit form to following:
Distinct('20190325_case13',Executor.DisplayName)
On your side, you should type:
Distinct('Executive Managers','EMG Name'.DisplayName)
Set the Update property of the PersonColumn Data card which contains the ComboBox control to following:
{
'@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims: "i:0#.f|membership|" & Lower(LookUp(Office365Users.SearchUser(),DisplayName=DataCardValue15.Selected.Result).Mail),
Department: "",
DisplayName: DataCardValue15.Selected.Result,
Email: LookUp(Office365Users.SearchUser(),DisplayName=DataCardValue15.Selected.Result).Mail,
JobTitle: "",
Picture: ""
}
On your side, you should type (set the Update property of the ExecMgr Data card (in your Edit form) which contains the cboExecMgr to following😞
{
'@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims: "i:0#.f|membership|" & Lower(LookUp(Office365Users.SearchUser(), DisplayName = cboExecMgr.Selected.Result).Mail),
Department: "",
DisplayName: cboExecMgr.Selected.Result,
Email: LookUp(Office365Users.SearchUser(), DisplayName = cboExecMgr.Selected.Result).Mail,
JobTitle: "",
Picture: ""
}
Note: I assume that the Person column in your Main SP List does not enable the "Allow multiple selections" option.
Please consider take a try with above solution, then check if the issue is solved.
Best regards,