Well, I have the same problem, struggle long time with it and found finally what that bugs happend and how you can make a workaround
First of all if you set only 1 Character in your scene to swimming and as example your Main Character is fallling (by spawning in the lvl by current camera location) it switch automatic from falling to swimming. Even if you start at “start point” if you jump and fall after it, it switch automatic to swimming.
The only workaround I could find until now is to disable “Can Swim” in the Character Movement Component of your BaseCharacter (Parent Class). Even it is disable you can set your Character to swim or not swim by setting the water volume and Set Movement Mode to swim
Hope that help you guys!
ps. Even in the Unreal Engine 5.0 its not fixed