Please check the solubilty of KH2PO4 (22.6 g / 100 ml water at 20C) and K2HPO4 (149 g / 100 ml water at 20C). Found at WIKI.

At pH 7.0 you are below the second pKa of fosforic acid, so the majority of your dissolved salt is KH2PO4.

When preparing the 50% w/w buffer at pH 7, there is probably some warming up of the solution, hence increased solubility. After cooling down the KH2PO4 will precipitate.

You must either use a lower concentration (for example 20% w/w) or a higher pH of your buffer.

Which type of K-Phosphate u use? The solubilities are mentioned above by Dr.Wielders. Check it please.