When an old world is loaded in 1.13.pre-1, the InhabitedTime variable is not being copied. This causes the Local Difficulty to be reset.

Load an old world in 1.12.2 and go to a location where the Local Difficulty is high. This is easiest to see on Hard. Press F3 and note the Local Difficulty.
Load the same world in 1.13.pre1. Press F3 and note the Local Difficulty. It is not the same.

Equivalent reproduction steps:

Inspect at the InhabitedTime for a chunk in an old world from 1.12.2 or earlier.
Load the world in 1.13.pre1.
Inspect at the InhabitedTime for the same chunk. It is reset to zero.