" I have a watering system connected to the battery so I know the cells have the water level required, "

Single point water systems do not "guarantee" all the cells have the "correct" water level. They are a time saver, but you still need to pull the battery periodically and manually check the water levels in each cell. Every 60 to 90 days is a good practice. These systems are notorious for underfilling as well as overfilling, especially as a battery ages and debris accumulates in the water. Overfilling is usually immediately evident. Underfilling isn't.
Without periodic inspection, you wouldn't know there was an underfilled cell until you have a problem similar to yours.