Lossless as in "lossless decomposition" means that :
- any relation value of the original schema
- * that satisfies all the dependencies in the original schema *
- can be decomposed by relational projection into relation values corresponding to the decomposed schemata
- and the natural join of those decomposed relation values is guaranteed to yield the original relation value back again.
That is what "lossless" means, and that's ALL that it means.
It has nothing to do with expressibility of FD's in decomposed schemas (which may indeed be "lost" by the decomposition - as is the case in your example).
There is no bug.