I had the same issue when recording a soloed track via AB so I assume it can happen any time that Cubasis is both the input and output within AB. (Which is how i presume you were carrying out the mixdown?) I rebooted Cubasis, the effects App and AB and the issue was resolved.
It looks like something in the recording connection can drop out on occasion. The strange bit is that you can hear AB working so the audio chain is ok it’s just that when you record you get a blank region.