Leave the pH alone and don't worry about it. With the chlorine level that high, the pH result won't be accurate anyway, but if you're just using bleach, the pH is not going to change enough to be an issue. Focus on your chlorine and forget about the rest for awhile.

If you "add more bleach for insurance", then you may just be wasting bleach, so I wouldn't do that. Test your water again, estimate the level, then add bleach if you're under the 15 ppm mark. (And if you did get to 21 ppm last night, it won't hurt the pool--it would just be more available chlorine to work on the algae).