If my CO2 switches off the pH in my reactor will creep up pretty quickly, not instantly mind you, but certainly noticeable (and sort of linear). Maybe 20-30 minutes to go fro 6.6. to 7ish and eventually it will be the same as the tank.
Mind you I don't read the pH inside the reactor itself (because I can't, I'd have to modify mine to do so), so I only read the effluent as it drips into the tank (my probe sits in an old pill bottle that the reactor drips into, and then overflows into the tank ... ghetto FTW!). But I don't think there would be a significant difference in pH readings because of this. Certainly not enough to think that 24 hours later it's still at 6.6.
What flowrate do you have for the reactor itself? (I have mine in the 60ml/min ballpark.)
Is it possible the probe is old and reading low, or could it be sitting in or near a pocket of undissolved CO2 or something?
I know I have a old probe measuring my tank. It's only good to show me that there is a pH high during daylight and a pH low during night, but otherwise the value it reads is way wrong. For example I just checked and it says my tank pH is 6.8. I'm reasonably certain that my tank is not, however, actually at 6.8 at the moment.
Recalibrating fixes this but it only stays correct for a couple days then it starts to drift down pretty fast. It would need to be recalibrated weekly basically, and that's too often for me. (That reminds me I need to replace that darn thing. My new one came a couple days ago but I haven't had time to swap it out.
)