I had a similar thing happen with one of my lead-acid batteries. Either using water or acid I filled cells to the proper level. Large glass doors allowed sunshine in the room. One or more cells overflowed. There was no apparent cause. My theory is that I overfilled just enough to cover the open slot in the lower rim of the plastic fill hole. This sealed the compartment. Then either hydrogen gas was generated, or air within expanded in the heat. Anyway it pushed fluid up the fill hole, and out of the battery housing. It was a one-time occurrence. I don't think it ruined the battery but I don't suppose it helped.
Look closely at the entire fill hole structure, in case you spot anything abnormal. Do you have a hygrometer? Test concentration in all the cells. Charge the battery. (L-A batteries prefer to be stored fully charged.) Test afterward with the hygrometer.