TL;DR
Make furnaces work with rainbow belts.What ?
Mixed rainbow belts can be fun. But currently it can make furnaces "stuck" with one item of input material, as they burn two at a time. There isn't a gameplay workaround that I'm aware of, besides "cleaning" the furnaces manually, or besides just waiting until the same type of ore appears at the same furnace. Rows of inactive stuck furnaces feel lame though.Possible workaround: if the furnace has been inactive, but has had input material, for e.g. a minute, burn the material even though there is only item. Or toss it out so the furnace can continue working on whatever ore is actually available.