There are still a lot of ways for the experiment to get the wrong result in vacuum. The force is really quite tiny.
Their "hard" vacuum was 1e-6 torr scale. That might still be enough to to produce some force. There can be effects from temperature causing anomolus forces in the suspension mechanism. There can be magnetic field effects from power cables. Torque from RF cables getting warm.
This would be a very difficult experiment to do correctly. Without details, it is much more likely that they got it wrong, than that they have found a violation of conservation of momentum.
As an aside, quantum mechanics still conserves energy and momentum. The pushing on virtual particles from the vacuum doesn't work unless you add enough energy to turn them into real particles - and then you just have a photon drive variant (which would produce far less thrust).