Hi @R3dKap,
Do you want to save a value from the Combo box (connected to another data source) into a Lookup column of your SP list within your custom form?
I have made a test, and the issue is confirmed on my side. I think this issue is related to the modification of data source the Combo box control connected to.
Currently, we could not save a value from the Combo box (connected to another data source) into a Lookup column of a SP list directly.
As an alternative solution, please take a try with the following workaround:

Set the OnStart property of the FormScreen1 to following formula:
ClearCollect(VAR_A,Filter('20180920_case7_1',EmailAddress=User().Email))
Note: The '20180920_case7_1' represents the Datasource_A that you mentioned.
Set the Items property of the Combo box control within the FavorColor Data card to following:
VAR_A
Set the Update property of the FavorColor Data card control to following formula:
{
'@odata.type' : "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference",
Id:LookUp(Colors,ColorName=DataCardValue4.Selected.Color,ID),
Value:DataCardValue4.Selected.Color
}
Note: The Colors represents the SP list data source which the FavorColor lookup column reference values from. The FavorColor column is a Lookup column in my SP list the current custom form connected to. The Color represents the column within your Datasource_A, which you use to display within the Combo box control.
On your side, you should type the following formula:
{
'@odata.type' : "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference",
Id:LookUp(ReferencedSPList,ReferencedColumnName=DataCardValueX.Selected.DisplayColumn,ID),
Value:DataCardValueX.Selected.DisplayColumn
}
Note: The ReferencedSPList represents the SP list your Lookup column reference values from. The ReferencedColumnName represents the column within the ReferencedSPList your Lookup column reference values from. In addition, please also make sure that the values that you selected from the Combo box control have been existed within the ReferencedSPList list.
The GIF screenshot as below:

Please also check and see if the following thread would help in your scenario:
https://powerusers.microsoft.com/t5/General-Discussion/Lookup-Field-Default-Value-Error/td-p/79481
Best regards,
Kris