Despite regular sandstone having hardness and blast resistances of 0.8, smooth sandstone has a blast resistance of 6 and hardness of 2. This is likely due to historical reasons, specifically as a result of hardness being inherited from slab blocks,[1] however the "correct"/"expected" values were changed to the "incorrect" values in Bedrock Edition.[2]
Smooth sandstone's block model has been removed, presumably on accident due to this block being unobtainable in normal gameplay, resulting in the missing model being used as a fallback.
Smooth sandstone exists as a metadata variant of the double slab block (block ID 43): if such a block had a metadata value from 8 to 15, it would use the top texture on all six sides. In practice, this affected stone slabs (creating smooth stone), sandstone slabs (creating this block) and quartz slabs (creating smooth quartz).