The creation of multi-rests happens automatically (the only exception being that you can set the minimum number of consecutive empty bars before they be converted to a multi-rest). But those bars really do have to be completely empty.
However, as you say, you can split a multi-rest manually (I didn’t realize from your original post that you had done so, or am I misunderstanding? 
), by right-clicking on the multi-rest and choosing Properties>“Split Multi-Rest”.
This creates a “Split” event, which, while normally hidden, can be made visible by checkmarking “Split Rests” in the Score Editor’s extended toolbar.
So, in your case, at least you’ll be able to see whether Cubase thinks this was a valid 16-bar multi-rest with a deliberate split at 8 bars (therefore you’ll see a “Split Rest” event), or if not, you can look for something else hidden at the end of bar 8 or the start of bar 9.