It could really be a number of things. It could be bacterial. Usually anaerobic bacteria. Like in a deep sand bed or sulfur reactor. C02 for example will lower ph but not affect alkalinity until lower than 5 ph ish. More what I was thinking adding acidic dose, or anything acidic. Rodi water after resins sometimes acidic, in mine it comes out at ph 5. Gases, amino acids, bunch of things. Along with something like boron or other things that add to alk that is not being used as fast as you dose it. Keeping ph in a down trend while alk staying the same. Same thing until a certain ph for other organic acids, formate, acetate...etc, and conjugate bases. Only thing I think other than aeration that could help quickly would be to buffer the dose container with something basic to get dose over 7-7.5 ish. Would take a drop or two of sodium carbonate, or calcium hydroxide, some playing around with ph meter to get it there. Some dilute acid if you over shoot and get precipitation. Also check rodi output. I use a old carbon reactor filled with aragonite to buffer it. Ph 5 in about 9 out. Will definitely take some troubleshooting.