Did you request the OFP or at the INIT page requested the init? Only then fueling and loading works, which would also be the correct order to program the MCDU. If that does not work your plane is not connected properly to your Simbrief account and you most likely did not enter your Simbrief user name.
Go see the beginners manuals at the Simbrief website and if you need support go to their official support channel- their discord server.
It definitely is not a bug.