This makes zero sense unless they are essentially abusively charged.
AGMs, which are also valve regulated lead acid batteries, are designed to fully recombine the formed hydrogen and oxygen inside. The “valve” prevents escape.
They can overpressurize on charging and lose gas. Recall that coulombs and amperes relay numbers of electrons. Electrochemical reactions are reliant upon mass - quantity of reactant to create those numbers of electrons in the circuit. If charged too fast, ie too many amperes, the overpotential (voltage) must be higher. This gives a one-two punch. Higher voltage which drives the electrolysis reaction, and more current which means more electrolysis can happen. Too much happening (faster splitting than recombination) will create pressure. Simple concept. Then the burped has, which should have recombined to water, is lost forever, impossible to add back.
So it is possible. But I’d suspect it needs to be a very fast charge on a very high impedance battery. In normal use, they should not lose appreciable water.