The Taylor test kit says the following about this:

FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operator mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

It is true that adding chlorine will make the pH rise and that when it gets consumed the pH will come back down, but the above effect may also be what is happening in your case. This is nothing to worry about.

Richard