The notion that spoken and written Lojban should be the same (have a one-to-one mapping.)
"Isomorphism" is a strong word. It requires not only that no information be lost in converting from writing to speech or the other way, but also that no information be gained. The two must be structurally identical, and therefore functionally identical (from a mathematical point of view) as well. mi'e jezrax
Breakdowns of audiovisual isomorphism are possible.