Considering the mRNA need the cationic lipid nanoparticles to help them get into a cell, if the mRNA is already uptake by RCB, the lipid nanoparticle that carried it will have fused with the RBC's cell membrane already, and the mRNA will stay inside the RBC forever until the red blood cell goes apoptosis and won't be able to transfect other cells as its cationic lipid shell is gone, unless it is repackaged into extracellular vesicles like exosomes which has much weaker transfection ability...based on what I understand.