Rivers not generating with water

This behavior is entirely expected since the river biome generates over sea level, and thus isn't flooded by the worldgen.

Cannot confirm in 1.13-pre10. The seed and coordinates are in my attached screenshot. The world was freshly created in 1.13-pre10.

For future reference:

Water only fills naturally generated areas at or under y=62 in a default world. Some rivers generate over y=62, and thus parts of them or maybe even the entire river may be found dried up. However, at or under y=62, the river is "flooded" as expected. Hope this helps!