When loading a world created prior to 1.13 in 19w02a, any chunk that was not loaded in 1.13 before will get reset to its original state, meaning that it will look like it would if the chunk was just discovered. Oddly enough, blocks which in the original state of the chunk would be filled with air will remain unaffected, resulting in parts of the modifications made by the player remaining.

Any chunk that was loaded in 1.13 in between loading it in an older version and loading it in 19w02a remains unaffected of this reset.

It's possible that this issue existed in prior 1.14 snapshots, though I didn't test weather or not this is the case, so I can't confirm weather or not this is the case.