Hello,
Sorry but yes this is a limitation: there is no back en forth conversion possible because in some case, e.g. when multiple slabs are used, there is no backward conversion possible. Thus we have decided to not invest time on that, even for the legacy conversion node in order to avoid spending time on potential bugs there.
Substrate is experimental and that non symmetrical conversion is clearly stated in our documentation. You should only experiment with substrate at this stage. Not executing cross project material creation/look-dev. Also because it is experimental, it is worth mentioning that we might change the interface without data conversion and break backward compatibility (material might become broken any release until we reach another development stage).
I hope this clarify the current experimental status of Substrate.