In MSFS 2024 A321-251N, I just edited and generated a flight in EFB SimBrief.
In the MCDU, I used INIT REQUEST to enter the plan. The Inibuilds A321-251N doesn’t support IMPORT PLAN directly into MCDU.
In Navigraph Hub, please try installing SimBrief EFB App v1.0.0-beta.1, restart machine, and try again. If still an issue please post SimBrief Route.
Please post a picture of your plan in the default planner after importing from SimBrief.
Cheers
Ian