Happens here in Cubase Pro 15.0.21.
Works in sync within empty project.
Sync fails in other projects with few instrument tracks and fx plugins.
That means, the beat within Pattern event editor of GA 6 is lagging and then stumbling (when loop in Cubase is set). In opposite to the normal midi pattern or the style pattern modes.
Same result after converting midi/style pattern into Pattern event mode of GA6.
On the other hand, the Pattern Event track of Cubase itself is always in sync.
So - for me it’s not usuable in this state.