Selection of too high a pH could lead to voids developing in your column. Data for the dissolution rate of silica in pure aqueous solvents as a function of pH shows significantly increasing solubility above pH 7. If a void has developed in your column due to dissolution, you will observe significant peak shape distortion for every peak.
The Cogent Bidentate C18™ column is very hydrophobic and so it is difficult for water to reach the surface in order to dissolve the underlying substrate. Bulky protecting groups are also used in some materials to inhibit base from reaching the silica structure. In cases such as these, the upper pH limit is extended significantly. However, the Cogent Diamond Hydride™ has a small amount of bonded carbon and is more susceptible to dissolution than some other stationary phases.
However, in many cases it is not necessary to use a higher pH in aqueous normal phase chromatography. The most important thing to consider is the pKa of your analyte. This will determine the ionization state of the analyte.