For whatever reason, hamata seems to be prone to this. I haven't quite figured out what causes it, but it could be heat related (daytime, not nighttime). I've seen it occur on probably three different clones over the course of two years, and if memory serves me well, it always occurred in the summer.
At first, I began to think that the constant moisture on the crown could have caused problems relating to the water potential of the plant and therefore mass flow, which could theoretically cause these kinds of problems, but I've seen this happen in dry conditions.
I looked at this thread yesterday but decided not to mention this so as not to sound like an alarmist, but I would go with what pebes said. Although I personally wouldn't assume thrips if this was my plant (they usually decimate the growth point to where it looks black and crisp), I would check very, very carefully for mites (you will literally need magnification to see them). I don't know why, but hamata seems to be an island for these under certain conditions.
Your temperatures are fine for that species, by the way.