Try it with Mixcraft 10.5 build 618 (check for updates if you don't have it yet).
If it still happens with build 618, go into Mixcraft's General preferences, click the option to zip up the log files, then submit an Acoustica Support Request and attach the log files.
For those interested, after lots of trial and error I think I've found the culprit: the FUSER plugin. I had the kick sidechained into the bass to autocorrect phase. Strangely, the issue occured even when the plugin was deactivated. I deleted the effect entirely and now it seems to mixdown fine... drums audible.