if I set a custom speed on the hero, let's say 20, before going on a shallow water terrain type, the hero would slow down, which is normal, the engine runs a algorithm to slow down speed a bit on shallow water, but if you set another speed while on shallow water, the new speed is only set when you're not on the shallow water terrain, same thing happen while in water, you need to press action (swim) or go on another solid terrain to update the new speed
You can reproduce this glitch with the debug keys.
This is a bug, yes.