Seriously, where did all the flavor go?
(https://leafimg-a.akamaihd.net/default/ppds/aa16be74-a92a-4de0-8777-2933d783035e.jpg)
Are you microwaving it?
heat it slow on the stove and stir in some milk
Try it outside the microwave as an experiment. I'd guess it gets heated unevenly and some of the butter / milk / cream is either partially overcooked or doesn't get melted again.
It's possible that you're breaking the sauce. Does it kinda separate into a layer of oil and a layer of creamy stuff? A de-emulsified sauce will taste like shit.
I imagine it has something to do with the fats. Idk. Just a guess