Hmm any experience I have had with it... was with bettas who have had traumatic damage to their fins, via tearing, severe fin rot, and sometimes biting.
Most common that I have seen, is in severe fin rot cases... where the fins in sections rather than at the end deteriorate, causing the fins in the center to be thinner... when it heals, it heals awkwardly causing a bumpy or lifted appearance. This appearance, is the grow back - not the sign of the actual fin rot
It also, can be from hard water - or metals in the water. This is more common to curled fins, rather than the bumps/lifted areas, but it does happen.