Some Pumpkins losing faces after converting to 1.13

After converting a world from 1.12.2 to 1.13-pre9 some of placed pumpkins lost their faces, but others did not. It seems to be that pumpkins placed on grass blocks get converted to uncarved pumpkins, so that naturally generated pumpkins in 1.12 could get transformed to uncarved pumpkins in 1.13.