The whole idea of carnosine causing the formation of lipofuscin was, as far as I can tell, nothing more than a speculation in a paper by Alan Hipkiss. One of the hallmarks of molecular damage to a protein (due to oxidation or glycation) is the formation of inappropriate carbonyls. Carnosine is thought to react with these protein carbonyls, forming "carnosinylated" proteins, which marks them for elimination in some way. The most probable fate of such proteins would be proteolysis leading to soluble products. In order to form lipofuscin, the protein or some part of it would need to be rendered indigestible by proteolytic enzymes. Proteolytic enzymes (proteases) sever the peptide chain at the site of specific residues, for example, trypsin cleaves at lys or arg sites. This has the effect of cutting a protein up into a number of smaller peptides. These shorter peptides are cleaved by other peptidases, and are ultimately broken down into individual amino acids which can be recycled. I think it's unlikely that a small number of carnosine tags would completely prevent proteolysis, and once the protein begins to unravel, it becomes highly exposed to protease attack. Thus the only way I can see for the formation of lipofuscin is that a protein would have to be so badly oxidized that carnosine would label it to such an extent that it would be prevented from any proteolysis, and frankly such a messed up protein should have been eliminated long ago by normal housekeeping. The only other way I can see would be if a short carnosinylated peptide were itself to be the problem. Given the water solubility of peptides, this seems fairly unlikely. Dr. Hipkiss offers circumstantial evidence for the carnosine-lipofuscin hypothesis, in that the most lipofuscin is found in muscle, nerve and brain, tissues that are high in carnosine. However, those also happen to be the most long-lived cells, and short-lived cells are replaced before they accumulate much lipofuscin, so I don't think that is evidence at all. In the linked paper, Hipkiss presents rationale and evidence that carnosinylated proteins are degraded via the proteasome, perhaps even more effectively than oxidized proteins without carnosine, so I think he was just trying to cover all the possible fates of carnosinylated protein when he came up with the lipofuscin idea.
It seems to me that carnosine-induced lipofuscin formation is not likely on the basis of the physical properties of proteins and peptides, and that there isn't any observational evidence for it. If someone has run the experiment where animals are fed carnosine and lipofuscin is analyzed versus controls, that would really sort it out. As far as I'm aware, at this point it's a speculation that doesn't appear very likely to be the case.
