On Mac. It seems to be a legacy issue. I was testing this on an imported game built with beta 2. Started a new game and Debug Mode works fine!
When you have a chance please send the file to support@buildbox.com so we can work out the issue. Very strange though that it is happening as I don't think anything has really changed with debug mode. Thanks!
its fixed! Did a reboot since yesterday and now debug mode in Preview is working fine. Bit of a weird one that!