This is independent of the Minecart Experiments being enabled; with them disabled the bubble particles only appear on one side, making this even more "weird-looking" than with the Experiments.

Steps to reproduce (videos are attached):

1. Dig out a line that is 8 blocks long.
2. Place water on one side.
3. Place powered rails on the other side and activate them.
4. Place a lot of minecarts on a rail using command blocks (minecart cluster).
5. Push the minecarts in the direction of the water.
6. As soon as the minecart cluster touches the water, it sends bubble particles upstream.