It seems the purpose of this FMS is to select a different sales GL where the Series selected contains 'EX'
The FMS itself seems to clear the value when it does not contain EX.
Try adding an else clause:
Declare @Series as varchar(10)
set @Series = (SELECT Distinct(T0.[SeriesName]) FROM NNM1 T0 WHERE T0.[Series] = $[OINV.Series])
Case when @Series Like '%EX%' THEN Select '400000'
ELSE SELECT '400010'
END