Mobs that are standing normally on soul sand will not be propelled upwards by water as per the new functionality in 1.13

steps:

1. create a 4x4 tube of at least 5 blocks height with 4 soul sand at the bottom
2. place a mob (tested with zombies and skeletons) at the bottom of the tube
3. fill the tube with water starting from the top, ensure that all water blocks are source blocks

intended behaviour/ vanilla server behaviour:
mobs launch up to the surface of the water the instant water touching the soulsand below them becomes a source block

Actual behaviour/spigot behaviour:
mobs stay sunk at the bottom of the tube and are not propelled upwards.

educated guess as to cause: upwards force is only applied to mobs when they are floating

attached image contains:
right side was created in a vanilla minecraft server following above steps
left side was created in the spigot version listed above following the above steps

i can upload a video if requested.