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