I have mixed some coins just now, and I too received a voucher code instead of (a) private key(s). Is it because the reserve ran dry? Step 2 mentions that all chips are withdrawn.
So far, every time that you get a voucher instead of the private keys to your chips has been because of lack of chips. I assume that for some reason there is very high demand right now, as it is not usually this way. The only time this wasn't the case was back when the fork to Bitcoin Cash happened, which forced ChipMixer (and many other services) to close and wait for security purposes. You can always email them about it to be sure and try to save your session in case it takes longer than it usually does to top up the chips.
Maybe the high demand can be explained by the recent price drop/JC morgan interview/china's ICO ban => people getting scared, wanting to hide the fact that they own BTC, or maybe wanting to mix their BTC before withdrawing?
That's a good theory. If I were residing in China this would definitely be my move. After so much cryptocurrency-related banning in China, the people are probably expecting more. Maybe they're expecting the Chinese government to "ban" bitcoin. again.
Damn though. I knew mixing is being used alot, but I didn't know the demand is THIS high.