Sign doesn't stop flowing water, only removes the source when placed on it. (not sure if this is on purpose)

I was just playing on a server when suddenly the owner said 'Awww, I don't think signs stop water anymore'. So I went to a small water hole and placed a sign and it removed the source, like normal. However, when I placed it on the flowing water, it removed that block, but the water continued to flow on the other side of the sign.