Should I be concerned about this? Is this standard practice with them?
I wouldn't be concerned, it's not very usual for Bitmain to delay shipping, they have been known to be very accurate with the shipping aspect, they could very well send you a dead machine, or a machine that works for a few days or weeks and then dies, they might even send you a tube that looks like a miner but when you open it you find a dozen of heatsinks that need soldering, but I am pretty confident they won't scam you.
With that being said things are not looking pretty for Bitmain, they had a "
hard fork" and this time it is a serious one, so I wouldn't be surprised if they ship you nothing for months to come, meanwhile everyone should be careful dealing with Bitmain until this war between Jihan and Zhan is over.