Why does the roux break down in my chicken velouté sauce?
Per SAJ14SAJ's comment, the suspension in a traditional veloute relies on the gelatin to keep everything in place. Stocks are naturally rich in gelatin, due to the bones and connective tissue used to make them. Using a base to make the sauce is probably not providing the gelatin necessary. Other rich liquids frequently do separate when simmered for a long time (think curries and gumbo), so it's to be expected that your emulsion will break down eventually. I think your solution is to either use a real stock instead of the base, or perhaps fortify your liquids with a bit of gelatin before adding the roux (or barring those, cook only until your starch is gelatinized).