Why do you use acetic acid instead of water to create your standard riboflavin solutions?
From Sigma product information: 


  The flavins are light sensitive and unstable in alkaline solutions.  The dry solid is stable to diffuse light, but is highly photolabile in solution, especially in alkaline solutions. Neutral and acidic solutions are stable in the dark (3% decomposition per month at 27 °C at  pH 6.0).  Acidic solutions may be sterilized by autoclaving. 


So making a slightly acidic solution probably helps to prevent decomposition. If you dissolve it in pure water without a buffer, the pH is not well defined.