Hello,
same Prob here. I get no answer either from support or from forum.
It seem´s to me general midiclock prob. Because Miditempo is calculated, so it must have a few bars to calculat the bpm. But thats not the reason for synced loop desaster alone. I never get a stable loop with extern Equipment. the only workaround for is to set the bpm the extern device and disable the clockout in Cubase. Then record the loop and turn the clock on again.