Hi @Anonymous,
Have you solved your problem?
Could you get the Mail property (CF_QCM_Data.Selected.Mail) based on the formula within the Items property of your Combo Box control (CF_QCM_Data)?
Based on the formula that you provided, I think there is something wrong with it. In addition, I think it is not necessary to initialize a context variable to achieve your needs.
I have made a test on my side, please take a try with the following workaround:'

Set the Items property of the Combo Box control (on your side, it is CF_QCM_Data) to following:
Office365Users.SearchUser()
On your side, you should type:
Filter(
Office365Users.SearchUser({searchTerm:"", top:500}),
JobTitle = "Quality Control Manager" && Department = CF_Project_Data.Selected.Title
)
Set the Update property of the Data card (On your side, it is CF_QCM) which contains the Combo Box control to following:
{
'@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims: "i:0#.f|membership|" & Lower(DataCardValue8.Selected.Mail),
DisplayName: DataCardValue8.Selected.DisplayName
}
On your side, you should type:
{
'@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims: "i:0#.f|membership|" & Lower(CF_QCM_Data.Selected.Mail),
DisplayName: CF_QCM_Data.Selected.DisplayName
}
Please also check and see if the following blog would help in your scenario:
https://powerapps.microsoft.com/en-us/blog/default-values-for-complex-sharepoint-types/
If you have solved this issue, please consider click "Accept as Solution" to identity this thread has been solved.
Best regards,
Kris