All lead acid batteries share certain charge characteristics. VRLA AGM batteries are no exception.
During the charge cycle, hydrogen and oxygen are produced. In VRLA batteries, these gasses are allowed to build up pressure--to a point. This allows the oxygen and hydrogen to recombine and form water. That's what makes VRLA batteries "maintenace free". If the charge rate, and discharge amount are not carefully regulated, the pressure build up exceeds safe pressures and the valve vents the excess pressure.
That's where your water went.
Either the discharge levels went too deep a few times, or the charge protocol that the inverter is using does not use an appropriate protocol for your particular AGM.
If the charge rate is faster than optimal for your pack size/brand/type, this will cause outgassing and loss of water.
If it doesn't monitor battery temperature, it's not using a sophisticated charge protocol. This will result in over or undercharging and shortened life.
If it doesn't have a very specific charge protocol for your specific brand of AGM VRLA battery chemistry, it's not using a smart protocol.
Here's a quote from a good battery page:
"Using normal target voltages to charge a battery that is colder than approximately 25ºC (77ºF) will result in an undercharged battery, which will deliver lower performance, reduced life and a higher life cycle cost. Applying normal target voltages to a battery that is hotter than 25ºC may result in an overcharged battery. This condition could lead to the drying out of VRLA battery cells. With flooded cells, the result will be excessive outgassing, increased battery maintenance in the form of more frequent watering and reduced battery life due to thermal stress. In fact, some battery manufacturers and charger manufacturers recommend not charging a battery that is 50ºC (122ºF) or hotter."
I would highly recommend a state of charge meter for your battery pack:
This gives you another (and better) method to determine if you are charging or discharging too fast, too deep, proper equalizing, etc.
Another potential problem is if one battery gets hotter than the others, it will charge differently and fail sooner. Periodic careful equalizing minimizes this effect.
Sometimes the first or last battery in a string of batteries suffers more than the rest of the string. Complicated...