Yes! Bleach loses potency over time. The higher the temperature and the higher the concentration the faster it decays. At 70 degrees 12.5% is only good for six to twelve weeks, at higher temperatures it will decay much more quickly.
The chart at the bottom of this link gives the half-life of chlorine at various concentrations and temperatures FOR HIGH QUALITY CHLORINE (lower quality chlorine will degrade faster). 12.5% chlorinating liquid at even 90F will have a half-life of around 70 days. At 75F, the half-life is around 220 days. Remember that these are half-lifes so the time it takes to lose half its potency and the loss will be faster early on as the concentration is higher. At 90F, the drop from 12.5% to 10.0% occurs in about 22 days -- again, for high-quality chlorine.