Not sure what unit you have but here is a Manual see if that helps.

In the Points to Remember section it says, "With each flush, a small amount of water may be emitted from the upper supply group. A drain at the base of the FLUSHMATE®system will eliminate most of this water. Depending on usage, you may see some residual water (less than 1”) in the china holding tank. This is a normal occurrence."

I'd empty the tank of water and check whether an excessive amount of water appears to be "emitted from the upper supply group". If not, I'd occasionally take off the tank cover to see how quickly the water builds up in the tank (after all, it has been installed six years). If it seems excessive, I'd check whether the "drain at the base of the FLUSHMATE®system" is plugged up.

-- amateur