Why is the pH level of carbonation loss decreasing then increases afterwards?
At the beginning, there was titrated mostly just the citric acid and the equilibrium amount of carbonic acid.

After that, carbon dioxide slowly converted to carbonic acid and $\mathrm{pH}$ has been decreasing.

Finally, the long term process took dominance as dissolved carbon dioxide together with carbonic acid were  leaving solution as gaseous carbon dioxide and $\mathrm{pH}$ have been raising.