Yes, same here. Win 10, Cubase 10.0.10. LoopMash constantly freezes during mixdown. Normal mixdown unuseable.
By the way it also freezes if I click a second time on an already blinking pattern number while it waits for the next bar.
Why do you make an export and import? Simply create a group track. Send the LoopMash output to that group. Then create an audio track and assign the group as its input. When you click the record button on that target audio track the output of LoopMash will be recorded directly. You could push buttons in the LoopMash plugin and do a live mix 
By the way this works with any VSTi.
If you want to record all eight LoopMash outputs simultaneously you need to work with eight groups and eight audio tracks of course.