Thank you for report! But I think it works correct.
Gauges drop to zero because electric power is removed. When EPU comes online, electric power returns and HYD PRESS indicators shows pressure. No actuators were moving so the pressure did not decrease.
Even if you try to move a stick, for example, HYD PRESS B will lower, but then it will regain again, as windmilling engine will be enough to build up pressure.