Starting at 18w01a it appears the default singleplayer player data is not saved anymore. It used to be saved as Data > Player entry in the level.dat file. This data was used when a player opened a world they had not opened before.
It appears the Player tag is also not read anymore. This can be seen when opening the attached world Default player test.zip.
If this is intended at least the entry should be removed from previous worlds to reduce the file size and not possibly cause a conflict in future versions.