Stone from 1.12 does not stack with 1.13 stone and can't be sold to Stone Masons.

Any smelted or silk touched stone that was created in 1.12 cannot be traded to Stone Masons and does not stack with any stone created in 1.13.

In 1.12 stone produced through silk touch and smelting did not stack with each other ( MCPE-47907 MCPE-48421 MCPE-47684 ) this issue was fixed in 1.13.0, however it seemed to overlook the result that 1.12 stone neither be traded to villagers nor stack with post-1.13 stone. It does appear that 1.12 stone can still be crafted into other blocks  and used at a stone cutter.

There is a workaround, although not practical for users with vast quantities of stone, place the 1.12 stone and then mine it with silk touch.