I read recently that when leaves on a java fern turn black/brown and die off, it is a sign of a new growth cycle beginning for the plant. Can anyone confirm or deny this?
I can confirm this to the extent of my personal experience.
I experienced basically what you described shortly after placing a healthy specimen of java fern into my tank. After about 2 weeks it started what you've described. I was worried about it for a while but since there were so many of the plantlets that looked very healthy I just let it go. Eventually the "mother" plant started to regrow and I had "fernlets" growing everywhere.