I’m not positive, but I think this could be related to hardness / alkalinity. If your water has a relatively large amount of buffering capacity, you might not see a measurable swing, especially if you are measuring with a cheaper kit. Plant and fish density is also a factor. The more plants you have, the more CO2 they’re going to release during the dark cycle. Ditto for fish, but you also need to factor in their night time activity level, which is lower for most fish, thus less O2 need. In one of my tanks it took several weeks before I started to see a noticeable decline in PH (CO2) and I have VERY soft Seattle water. I assume it would have taken even longer with harder water.
PS - How are you diffusing the CO2?