The hashrate of a strings design is heavily dependent
of the supply voltage. You are sure this didn't change?
The PSU is 24V +- 1% not 7% and the power (edit: at the wall) is the same, while an increased voltage should have been increased the consumption too.
My strings are of 28 chips or 14 per 12V instead of 15 as in your case. The last chip is replaced from a diode to get the IOVDD (wasn't sure if IOVDD = VDD will work, so played safe) - this is the only part that may affect the overall per chip voltage ... in case the voltage drop on the diode decreases with the time, but then the effect of the temperature would be much more, which is not the case, so my (current) bet is on either CMQ (+-) resitors change in value (= less unused current for balancing) or some kind of resonance of the chip clocks