Hey, not a known issue as far as I am aware. It follows the simbrief fine for me and I landed my last flight within the past half an hour.
What steps did you take to get the simbrief plan into the FMGC? A photo of your cockpit set up showing PFD and AP settings would also help and I’ll try to assist.
The fact it is showing a straight line would suggest it hasn’t intercepted the flight plan to be able to enter nav mode. Likely an easy fix but I need to work out where you’ve gone wrong to tell you what to fix 😊