Same thing happened to me, so this is not an isolated situation!
Sent coins to the provided address. Waited for confirmation, once it was confirmed I went back to the website, just to see it went down (I'm used to this Chipmixer DOWN! situation).
After it got up and running again, I went to step 2 and Total Chips : 0.000
The coins were already withdraw!
I had 3 open sessions in 3 different browsers. Mozilla, Chrome and Edge.
The one in Chrome and Edge went through successfully - There were no problems, I managed to withdraw the chips!
But the one in Mozilla, were already withdrawn!