I have a specific travel mug that I use on occasion that will curdle the creamer. The creamer I used is only a few days old and it only happens in this mug. It's a stainless steel double walled with lid. I have no idea why it does this. When I use a normal mug, I have no issues.
One possible difference is that when you pour coffee (or any other hot liquid) into a ceramic mug, the liquid cools a fair bit (the heat is "lost" to heating the ceramic cup). The double-walled stainless mug is designed to lose as little heat as possible, and the inner wall has much less thermal mass than the ceramic. So, when you pour the creamer into the stainless cup, the coffee is hotter.
The simplest way to test if this is the case, would be to measure with an accurate thermometer. And the solution, then, would be to allow the coffee to cool some before adding the creamer.
Alternatively, temper in the creamer. To do this, put the creamer in the cup first, then add a little coffee, stir, add a little more coffee, stir, then add the rest. The idea is to slowly heat the creamer up to the coffee's temperature. This should prevent curdling, even if your coffee is near-boiling.
It could be some residue on the cup, but I doubt it, because stainless is fairly easy to clean. Also, it'd be hard to imagine enough residue to curdle the cream without also being very evident in the taste of the coffee. Stainless steel itself is pretty non-reactive, so its probably not a reaction with the coffee or cream.
Are you by any chance cleaning the mugs differently? If you're using something acidic (vinegar maybe) to clean the stainless mug then it might be residue causing the curdling.