If I place a trapdoor over the top of a water source block (or in the water source block itself) I cannot walk from it to an adjacent farmland or grass path block. It bumps me backwards, as if I'm trying to walk into a wall. However, I can walk onto a full-size block just fine, and if there is no water source block underneath then I can walk onto the farmland or grass path like normal. This does not seem to be affected by snow or carpet, either.

Also, when standing on the farmland or grass path next to the water source block, I am pushed towards it as if it is a moving water block. I am not sure that the two bugs are related but I am including it in here just in case.