Any ideas on this?

That's an odd one. Does it happen in all gears? Is it when pedalling and / or back-pedalling?

Have they routed the chain properly through the mech? Does the chain look the right width? More importantly is it the right length?

Typical elimination process:

Check whether the jockey wheel isn't worn out. Pucture below shows various amounts of wear ranging from "pretty much new", all the way to "what the hell is this?"


Check for stiff links in the chain.

Bent hanger or mech cage can be the cause of such problem. Hanger would probably have to be apparent and would also cause serious shifting problems. Check the cage very closely. Try applying pressure on it in the direction opposite to the one the chain slips in.

Not sure about SRAM, but some Shimano mechs have directional tension wheels and to my knowledge, the difference is in the angle of the cogs. Not sure if this could cause slipping, but worth checking out. The tension should have arrows indicating which way it's supposed to turn.

If none of this is causing the issue, I don't know what could. Can't see it happening because of a worn cassette and the chain is new. Can't imagine how effed would the chainring need to be in order to send the chain slipping off of the tension pulley (not sure if that's even possible). Only other possibility might be too much grit on the pulleys.