Aussieta's review of what the OP described (post #1 of this thread), including addition of muriatic acid to reduce pH of 8.0, looks to be accurate.
Well noted singingpond, and thank you. My post should have been more accurately stated that no additional chemicals were added once the OP's pH was at the 6.8ppm level. This however still does not explain the inconsistency of the pH level when the MA had been add prior to that at the 8.0 pH level, as noted by Aussieta above in comment #17. I will continue to also watch for further inconsistencies in OP's declines with chemical adds, so we can better advise the OP on how to possibly prevent the occurrence in the future. Have a wonderful day