Okay! I have news to report. The folks at RWU emailed back:
"They sometimes lose them early in grow-out, a few weeks after we move them from the larval tank. We don’t exactly know why. We had a few batches where this occurred. Most of our fish haven’t had this issue. The good news is that the older fish we have that lost their dorsal fin grew them back, but it takes a few months. "
"They sometimes lose them early in grow-out, a few weeks after we move them from the larval tank. We don’t exactly know why. We had a few batches where this occurred. Most of our fish haven’t had this issue. The good news is that the older fish we have that lost their dorsal fin grew them back, but it takes a few months. "