I've had a few players report that the splitter is removing resources on some occasions. Not been able to replicate but it came along with the November update. Thanks,
Furnace Splitter deleting resources after update
Make sure you update your server with the latest changes and latest Oxide that was released a little while ago.
So if you increase the stack size limit above 18 then it puts your items in slots that don't exist I would suggest limiting the plugin to not allow anymore then 16 slots leaving one spot for wood and one for coal.