Space matter furnace never give extra ore (tested with input 40.000 ore, as result I received 48.000). I see that this furnace has module slot, and I see "item-name.spaceblock-mining-prod-module", which can be placed into this slot, but there is no way to obtain this module.

This happens because the Space Matter Furnace and Space Matter Refinery both have their furn.result_inventory_size set to one, even though the 10% chance to produce 1 extra product is handled by Factorio as a second output.

So setting furn.result_inventory_size=2 in both the space matter furnace and space matter refinery in data-final-fixes.lua fixes this problem!

Fixed this issue in additional mod: https://mods.factorio.com/mod/spaceblock-fixes.
Hope it helps to play now before it will be upstreamed.