Why is there a high current in the device even if the device is functionally working if VCCIO is powered before Vcc?
The power supply sequence should be strictly followed to ensure proper device operation. One of the reasons the device requires the sequence of power-up is because the device needs to configure/be configured during power-ups. The VCC powers up first to allow the internal configuration to be reset and initialized properly.
If VCCIO powers up early, it could cause the configuration not to reset in a proper state. When the device is not configured properly, there could be contention in the FPGA logic that can cause DC current. Even if the device appears to be functioning properly to the application, some of the contentions could be in an area that is not used in the application and not observed as a functional failure. Device and Design reliability will be highly compromised if the Power-up Supply Sequence is not followed.