If you place a boat in the water and try to walk on it, your movement is slowed and stuttered, and very not smooth.

But when the boat is on ground, I guess when it has no movement, not even the tiniest micro movement, walking on it is perfectly fine and works as expected.

The same bug happens when walking on a shulker's top while it's opening, and a bit while it's closing (not when in still closed or opened position).